Effect of selenium supplementation on thyroid hormone metabolism in an iodine and selenium deficient population

Thyroid dysfunction due to trace element deficiency-not only selenium but also zinc

INTRODUCTION

Levels of serum selenium (Se) and zinc (Zn) decrease when total parental nutrition (TPN) is administered without trace element supplementation for just a few weeks. These trace elements are involved in thyroid hormone metabolism and their deficiencies cause thyroid dysfunction. However, there have been few reports on the details of its clinical course.

CASE PRESENTATION

A 50-year-old man presented with thyroid dysfunction due to Se and Zn deficiency. He had an approximately 70-cm residual small intestine after undergoing intestinal resection and he received TPN without trace element supplementation for one and a half months. Blood tests revealed high levels of thyroid-stimulating hormone (TSH) and free thyroxine (FT4) and low levels of free triiodothyronine (FT3). An abnormal pattern of thyroid function led to suspicion of Se deficiency. Se supplementation raised FT3 levels and lowered FT4 levels to within their respective reference ranges; however, subclinical hypothyroidism persisted with transient TSH elevation. We suspected that Zn deficiency also contributed to the hypothyroidism and, therefore, initiated Zn supplementation, which resulted in normalization of thyroid function.

DISCUSSION

Although thyroid dysfunction has been reported in many studies conducted on Se and Zn deficiencies, hormonal patterns vary between reports. Further accumulation of cases, including detailed data on nutritional status, would be of benefit to elucidate the clinical reality.

CONCLUSION

It is important to consider Se and Zn deficiencies when TSH and FT4 levels are elevated. It should also be noted that transient TSH elevation may be observed with Se supplementation.

Selenium, Iodine and Iron-Essential Trace Elements for Thyroid Hormone Synthesis and Metabolism

The adequate availability and metabolism of three essential trace elements, iodine, selenium and iron, provide the basic requirements for the function and action of the thyroid hormone system in humans, vertebrate animals and their evolutionary precursors. Selenocysteine-containing proteins convey both cellular protection along with H₂O₂-dependent biosynthesis and the deiodinase-mediated (in-)activation of thyroid hormones, which is critical for their receptor-mediated mechanism of cellular action. Disbalances between the thyroidal content of these elements challenge the negative feedback regulation of the hypothalamus-pituitary-thyroid periphery axis, causing or facilitating common diseases related to disturbed thyroid hormone status such as autoimmune thyroid disease and metabolic disorders. Iodide is accumulated by the sodium-iodide-symporter NIS, and oxidized and incorporated into thyroglobulin by the hemoprotein thyroperoxidase, which requires local H₂O₂ as cofactor. The latter is generated by the dual oxidase system organized as 'thyroxisome' at the surface of the apical membrane facing the colloidal lumen of the thyroid follicles. Various selenoproteins expressed in thyrocytes defend the follicular structure and function against life-long exposure to H₂O₂ and reactive oxygen species derived therefrom. The pituitary hormone thyrotropin (TSH) stimulates all processes required for thyroid hormone synthesis and secretion and regulates thyrocyte growth, differentiation and function. Worldwide deficiencies of nutritional iodine, selenium and iron supply and the resulting endemic diseases are preventable with educational, societal and political measures.

Deiodinases control local cellular and systemic thyroid hormone availability

Iodothyronine deiodinases (DIO) are a family of selenoproteins controlling systemic and local availability of the major thyroid hormone l-thyroxine (T4), a prohormone secreted by the thyroid gland. T4 is activated to the active 3,3'-5-triiodothyronine (T3) by two 5'-deiodinases, DIO1 and DIO2. DIO3, a 5-deiodinase selenoenzyme inactivates both the prohormone T4 and its active form T3. DIOs show species-specific different patterns of temporo-spatial expression, regulation and function and exhibit different mechanisms of reaction and inhibitor sensitivities. The main regulators of DIO expression and function are the thyroid hormone status, several growth factors, cytokines and altered pathophysiological conditions. Selenium (Se) status has a modest impact on DIO expression and translation. DIOs rank high in the priority of selenium supply to various selenoproteins; thus, their function is impaired only during severe selenium deficiency. DIO variants, polymorphisms, SNPs and rare mutations have been identified. Development of DIO isozyme selective drugs is ongoing. A first X-ray structure has been reported for DIO3. This review focusses on the biochemical characteristics and reaction mechanisms, the relationships between DIO selenoproteins and their importance for local and systemic provision of the active hormone T3. Nutritional, pharmacological, and environmental factors and inhibitors, such as endocrine disruptors, impact DIO functions.

Relationship between dietary selenium intake and serum thyroid function measures in U.S. adults: Data from NHANES 2007-2012

Although numerous studies have explored the relationship between selenium intake and thyroid diseases, few epidemiological studies have investigated the association between selenium intake and thyroid hormones. Therefore, we conducted this analysis to investigate the association between dietary selenium intake and thyroid hormones. Our sample included 5,575 adults (age ≥ 20) years from the National Health and Nutrition Examination Survey (NHANES) 2007-2012. Thyroid hormones, including total triiodothyronine (T3), total thyroxine (T4), free T3 (FT3), free T4 (FT4), and thyroid-stimulating hormone (TSH), were detected. Multivariable linear regression models showed that log10-transformed selenium intake (LogSe) was negatively correlated with TT4 (β = -0.383, 95% CI: -0.695, -0.070) and TT4/TT3 (β = -0.003, 95% CI: -0.006, -0.0004) in U.S. adults. Besides, additional stratified analyses by sex demonstrated that LogSe was negatively associated with TT4 (β = -0.007, 95% CI: -0.013, -0.001) and TT4/TT3 (β = -0.664, 95% CI: -1.182, -0.146) and positively associated with FT4/TT4 (β = 0.031, 95% CI: 0.004, 0.059) in male adults. Meanwhile, subgroup analysis by iodine status showed that LogSe was negatively associated with TT4 (β = -0.006, 95% CI: -0.011, -0.002), FT4/FT3 (β = -0.011, 95% CI: -0.023, -0.00002) and TT4/TT3 (β = -0.456, 95% CI: -0.886, -0.026) in iodine sufficiency but not in iodine deficiency adults. Our results demonstrated that the increased dietary selenium intake was negatively correlated with TT4 and TT4/TT3 in U.S. adults. Furthermore, the association between dietary selenium intake and thyroid hormones was more pronounced in males and iodine sufficiency adults.

Trace elements and the thyroid

Trace elements, such as iodine and selenium (Se), are vital to human health and play an essential role in metabolism. They are also important to thyroid metabolism and function, and correlate with thyroid autoimmunity and tumors. Other minerals such as iron (Ir), lithium (Li), copper (Co), zinc (Zn), manganese (Mn), magnesium (Mg), cadmium (Cd), and molybdenum (Mo), may related to thyroid function and disease. Normal thyroid function depends on a variety of trace elements for thyroid hormone synthesis and metabolism. These trace elements interact with each other and are in a dynamic balance. However, this balance may be disturbed by the excess or deficiency of one or more elements, leading to abnormal thyroid function and the promotion of autoimmune thyroid diseases and thyroid tumors.The relationship between trace elements and thyroid disorders is still unclear, and further research is needed to clarify this issue and improve our understanding of how trace elements mediate thyroid function and metabolism. This paper systematically reviewed recently published literature on the relationship between various trace elements and thyroid function to provide a preliminary theoretical basis for future research.

The Nutritional Supply of Iodine and Selenium Affects Thyroid Hormone Axis Related Endpoints in Mice

Selenium and iodine are the two central trace elements for the homeostasis of thyroid hormones but additional trace elements such as iron, zinc, and copper are also involved. To compare the primary effects of inadequate intake of selenium and iodine on the thyroid gland, as well as the target organs of thyroid hormones such as liver and kidney, mice were subjected to an eight-week dietary intervention with low versus adequate selenium and iodine supply. Analysis of trace element levels in serum, liver, and kidney demonstrated a successful intervention. Markers of the selenium status were unaffected by the iodine supply. The thyroid gland was able to maintain serum thyroxine levels even under selenium-deficient conditions, despite reduced selenoprotein expression in liver and kidney, including deiodinase type 1. Thyroid hormone target genes responded to the altered selenium and iodine supply, whereas the iron, zinc, and copper homeostasis remained unaffected. There was a notable interaction between thyroid hormones and copper, which requires further clarification. Overall, the effects of an altered selenium and iodine supply were pronounced in thyroid hormone target tissues, but not in the thyroid gland.

Environmental Factors Affecting Thyroid-Stimulating Hormone and Thyroid Hormone Levels

Thyroid hormones are necessary for the normal functioning of physiological systems. Therefore, knowledge of any factor (whether genetic, environmental or intrinsic) that alters the levels of thyroid-stimulating hormone (TSH) and thyroid hormones is crucial. Genetic factors contribute up to 65% of interindividual variations in TSH and thyroid hormone levels, but many environmental factors can also affect thyroid function. This review discusses studies that have analyzed the impact of environmental factors on TSH and thyroid hormone levels in healthy adults. We included lifestyle factors (smoking, alcohol consumption, diet and exercise) and pollutants (chemicals and heavy metals). Many inconsistencies in the results have been observed between studies, making it difficult to draw a general conclusion about how a particular environmental factor influences TSH and thyroid hormone levels. However, lifestyle factors that showed the clearest association with TSH and thyroid hormones were smoking, body mass index (BMI) and iodine (micronutrient taken from the diet). Smoking mainly led to a decrease in TSH levels and an increase in triiodothyronine (T3) and thyroxine (T4) levels, while BMI levels were positively correlated with TSH and free T3 levels. Excess iodine led to an increase in TSH levels and a decrease in thyroid hormone levels. Among the pollutants analyzed, most studies observed a decrease in thyroid hormone levels after exposure to perchlorate. Future studies should continue to analyze the impact of environmental factors on thyroid function as they could contribute to understanding the complex background of gene-environment interactions underlying the pathology of thyroid diseases.

Thyroid function in patients with selenium deficiency exhibits high free T4 to T3 ratio

Selenium, one of the essential trace minerals, is present in vivo in form of selenoproteins. Iodothyronine deiodinase, a selenoprotein, is involved in the activation and inactivation of thyroid hormone. Therefore, patients with selenium deficiency may present changes in thyroid hormone levels due to inhibition of T4 to T3 conversion; however, this assumption is still under debate. In the present study, we retrospectively investigated the thyroid function in 22 patients with selenium deficiency. Thyroid stimulating hormone (TSH) and free T4 (FT4) levels were increased in 3 (14%) and 5 (23%) patients, respectively, and free T3 (FT3) levels were decreased in 6 (27%) patients. The FT4/FT3 ratio was significantly higher in patients with selenium deficiency than that in the control group. There appeared to be a positive correlation between the decreased rate of selenium levels and FT4/FT3 ratio, thereby indicating that patients with severe selenium deficiency also exhibited abnormal thyroid hormone levels. Furthermore, when selenium was supplemented in seven patients with abnormal thyroid hormone levels, the TSH, FT4, and FT4/FT3 ratio were significantly decreased and FT3 levels were increased. Collectively, patients with selenium deficiency could present the characteristics of not only low FT3 but also high FT4 and FT4/FT3 ratio.

Selenium deficiency risks in sub-Saharan African food systems and their geospatial linkages

Selenium (Se) is an essential element for human health. However, our knowledge of the prevalence of Se deficiency is less than for other micronutrients of public health concern such as iodine, iron and zinc, especially in sub-Saharan Africa (SSA). Studies of food systems in SSA, in particular in Malawi, have revealed that human Se deficiency risks are widespread and influenced strongly by geography. Direct evidence of Se deficiency risks includes nationally representative data of Se concentrations in blood plasma and urine as population biomarkers of Se status. Long-range geospatial variation in Se deficiency risks has been linked to soil characteristics and their effects on the Se concentration of food crops. Selenium deficiency risks are also linked to socio-economic status including access to animal source foods. This review highlights the need for geospatially-resolved data on the movement of Se and other micronutrients in food systems which span agriculture-nutrition-health disciplinary domains (defined as a GeoNutrition approach). Given that similar drivers of deficiency risks for Se, and other micronutrients, are likely to occur in other countries in SSA and elsewhere, micronutrient surveillance programmes should be designed accordingly.

Selenium in thyroid disorders - essential knowledge for clinicians

In the 1990s, selenium was identified as a component of an enzyme that activates thyroid hormone; since this discovery, the relevance of selenium to thyroid health has been widely studied. Selenium, known primarily for the antioxidant properties of selenoenzymes, is obtained mainly from meat, seafood and grains. Intake levels vary across the world owing largely to differences in soil content and factors affecting its bioavailability to plants. Adverse health effects have been observed at both extremes of intake, with a narrow optimum range. Epidemiological studies have linked an increased risk of autoimmune thyroiditis, Graves disease and goitre to low selenium status. Trials of selenium supplementation in patients with chronic autoimmune thyroiditis have generally resulted in reduced thyroid autoantibody titre without apparent improvements in the clinical course of the disease. In Graves disease, selenium supplementation might lead to faster remission of hyperthyroidism and improved quality of life and eye involvement in patients with mild thyroid eye disease. Despite recommendations only extending to patients with Graves ophthalmopathy, selenium supplementation is widely used by clinicians for other thyroid phenotypes. Ongoing and future trials might help identify individuals who can benefit from selenium supplementation, based, for instance, on individual selenium status or genetic profile.

A 2018 European Thyroid Association Survey on the Use of Selenium Supplementation in Hashimoto's Thyroiditis

OBJECTIVE

To investigate clinical practice regarding the use of selenium supplementation in patients with Hashimoto's thyroiditis (HT) among members of the European Thyroid Association (ETA).

METHODS

ETA members were invited to participate in an online survey investigating the use of selenium supplementation across the spectrum of benign thyroid diseases. Of 872 invited members, 242 (28%) completed the survey. After exclusion of basic scientists and non-European members, survey data from 212 respondents were eligible for further analyses. Responses from 65 (31%) individuals who did not at all recommend selenium, or only considered its use in the setting of a clinical trial, were not included in the final analysis of survey data from 147 respondents.

RESULTS

While only a minority of respondents (29 of 147, 20%) stated that the available evidence warrants the use of Se in patients with HT, a statistically significant majority (95 of 147; 65%, p < 0.001) used Se occasionally or routinely. Se was predominantly recommended for patients with HT not receiving LT4 (102 of 147; 69%) to reduce circulating thyroid autoantibody levels. Very few respondents routinely recommended Se to pregnant patients with HT.

CONCLUSIONS

A minority of responding ETA members stated that the available evidence warrants the use of Se in HT, but a majority recommended it to some extent, especially to patients not yet receiving LT4. This is questionable, and selenium is not recommended to patients with HT according to current ETA guidelines. Ongoing and future trials may lead to the reversal of current medical practice.

Micronutrients, iodine status and concentrations of thyroid hormones: a systematic review

Context

The metabolism of thyroid hormones, which are essential for normal development, involves many proteins and enzymes. It requires iodine as a key component but is also influenced by several other micronutrients, including selenium, zinc, iron, and vitamin A.

Objective

This systematic review was designed to investigate the effect of micronutrient status and supplementation on iodine status and thyroid hormone concentrations.

Data Sources

Using the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analysis) guidelines, electronic databases were searched from their inception to April 2016.

Study Selection

Human studies published in English and reporting data on micronutrient status and iodine status and/or thyroid hormone concentrations were included. Studies that examined the effect of micronutrient supplementation on iodine status and/or thyroid hormone concentrations were also included.

Data Extraction

A predesigned and piloted data extraction form was used to compile data from individual studies.

Results

A total of 57 studies were included: 20 intervention studies and 37 observational studies. Although observational evidence suggests that concentrations of selenium, zinc, and iron are positively associated with iodine status, data from randomized controlled trials fail to confirm this relationship.

Conclusions

Further studies are needed to provide greater understanding of the role of micronutrient status in iodine nutrition and thyroid function to ascertain the public health implications for populations worldwide.

Role of Selenium Intake for Risk and Development of Hyperthyroidism

PURPOSE

To investigate the importance of dietary selenium (Se) for hyperthyroidism.

METHODS

We performed a more in-depth analysis of a large cross-sectional study of 6152 participants from two counties within the Shaanxi Province, China. These counties are characterized by different habitual Se intake. We investigated the effects of a different dietary Se supply (0.02, 0.18, 0.6, or 2.0 ppm Se) on disease development in a mouse model of Graves disease (GD).

RESULTS

The cross-sectional study revealed a comparable prevalence of hyperthyroidism, irrespective of Se intake, in both counties. However, an unexpected sex-specific difference was noted, and Se deficiency might constitute a risk factor for hyperthyroidism, especially in males. In a mouse model, pathological thyroid morphology was affected, and greater Se intake exerted some protecting effects on the pathological distortion. Circulating thyroid hormone levels, malondialdehyde concentrations, total antioxidant capacity, and the titer of GD-causing TSH receptor autoantibodies were not affected by Se. Expression analysis of the transcripts in the spleen indicated regulatory effects on genes implicated in the immune response, erythropoiesis, and oxygen status. However, the humoral immune response, including the CD4/CD8 or T-helper 1/T-helper 2 cell ratio and the concentration of regulatory T cells, was similar between the experimental groups, despite the difference in Se intake.

CONCLUSIONS

Our data have highlighted a sexual dimorphism for the interaction of Se and thyroid disease risk in humans, with indications of a local protective effects of Se on thyroid gland integrity, which appears not to be reflected in the circulating biomarkers tested.

Selenium inadequacy hampers thyroid response of young children after iodine repletion

Selenium (Se) is an integral component of iodothyronine deiodinase, glutathione peroxidase and thioredoxin reductase enzymes and thus is important for normal thyroid function. This study investigated the influence of Se inadequacy on thyroid response of iodine-replete young children. Serum thyroxine (T₄), triiodothyronine (T₃), thyroglobulin (Tg), thyroid stimulating hormone (TSH), and Se were analyzed in 54-60 mo old children (n = 628) from the Amhara region of Ethiopia before salt iodization was commenced; analyses were repeated (n = 555) 15 mo after iodized salt became available. Iodized salt coverage increased from 12.2% to 91.6% of households. Median urinary iodine concentration (UIC) among children increased from 9 μg/l to 167 μg/l (p < 0.001). In addition, all thyroid indices except T₃ showed significant improvement (p < 0.05). Nearly, half of the study children (49.1%) had Se inadequacy (serum Se < 70 μg/l). Serum Se was significantly correlated with T₃ (r = 0.38, p < 0.001), T₄ (r = 0.15, p < 0.001), TSH (r=-0.205, p < 0.001) and Tg (r= -0.11, p < 0.01) concentrations 15 mo after iodine repletion; baseline serum Se and T₄(r= -0.22, p < 0.01) were inversely correlated. Despite adequate iodine status, children with low serum Se had lower serum T₄ (p = 0.003) and T₃(p < 0.001) but higher TSH concentration (p = 0.003). In the partial least square regression model, Se was among the latent variables significantly explaining T₄ and T₃. Results of the present study suggest that Se inadequacy negatively affects the thyroid metabolism of iodine-replete children and may present a substantial public health concern thus emphasize the need to consider correction of Se status for normal thyroid function as well as for benefits from its diverse biological roles.

Selenium and Selenoproteins in Immune Mediated Thyroid Disorders

Selenium is an essential micronutrient that is required for the synthesis of selenocysteine-containing selenoproteins, processing a wide range of health effects. It is known that the thyroid is one of the tissues that contain more selenium. The “selenostasis” maintenance seems to contribute to the prevention of immune mediated thyroid disorders. Prospective, observational studies, randomized, controlled studies evaluating selenium supplementation, and review articles that are available in Medline and PubMed have undergone scrutiny. The differences concerning methodology and results variability have been analyzed. Several authors support the idea of a potential efficacy of selenium (mainly selenomethionine) supplementation in reducing antithyroperoxidase antibody levels and improve thyroid ultrasound features. In mild Graves’ orbitopathy, selenium supplementation has been associated with a decrease of the activity, as well as with quality of life improvement. Future research is necessary to clearly understand the selenium supplementation biologic effects while considering the basal selenium levels/biomarkers, selenoprotein gene polymorphisms that may be involved, underlying comorbidities and the major clinical outcomes.

Effects of Selenium Supplementation on Graves' Disease: A Systematic Review and Meta-Analysis

Low selenium status is associated with increased risk of Graves' disease (GD). While several trials have discussed the efficacy of selenium supplementation for thyroid function, in GD patients, the effectiveness of selenium intake as adjuvant therapy remains unclear. In this systematic review and meta-analysis, we aimed to determine the efficacy of selenium supplementation on thyroid function in GD patients. Two reviewers searched PubMed, Web of Science, the Cochrane Central Register of Controlled Trials, and four Chinese databases for studies published up to October 31, 2017. RCTs comparing the effect of selenium supplementation on thyroid hyperfunction in GD patients on antithyroid medication to placebo were included. Serum free thyroxine (FT4), free triiodothyronine (FT3), thyrotrophic hormone receptor antibody (TRAb), and thyroid-stimulating hormone (TSH) levels were assessed. Ten trials involving 796 patients were included. Random-effects meta-analyses in weighted mean difference (WMD) were performed for 3, 6, and 9 months of supplementation and compared to placebo administration. Selenium supplementation significantly decreased FT4 (WMD=-0.86 [confidence interval (CI)-1.20 to -0.53]; p=0.756; I2=0.0%) and FT3 (WMD=-0.34 [CI-0.66 to -0.02]; p=0.719; I2=0.0%) levels at 3 months, compared to placebo administration; these findings were consistent at 6 but not 9 months. TSH levels were more elevated in the group of patients taking selenium than in the control group at 3 and 6, but not 9 months. TRAb levels decreased at 6 but not 9 months. At 6 months, patients on selenium supplementation were more likely than controls to show improved thyroid function; however, the effect disappeared at 9 months. Whether these effects correlate with clinically relevant measures remains to be demonstrated.

Severely low serum magnesium is associated with increased risks of positive anti-thyroglobulin antibody and hypothyroidism: A cross-sectional study

Trace elements, such as iodine and selenium, are closely related to autoimmune thyroiditis and thyroid function. Low serum magnesium is associated with several chronic diseases; however, its associations with autoimmune thyroiditis and thyroid function are unclear. We investigated the relationships between low serum magnesium, autoimmune thyroiditis, and thyroid function in 1,257 Chinese participants. Demographic data were collected via questionnaires, and levels of serum thyroid stimulating hormone, anti-thyroid peroxidase antibody, anti-thyroglobulin antibody (TGAb), free thyroxine, serum magnesium, serum iodine, and urinary iodine concentration were measured. Participants were divided into serum magnesium level quartiles (≤0.55, 0.551-0.85, 0.851-1.15, and >1.15 mmol/L). The median serum magnesium level was 0.89 (0.73-1.06) mmol/L; levels ≤0.55 mmol/L were considered severely low (5.9% of participants). The risks of TGAb positivity and Hashimoto thyroiditis (HT) diagnosed using ultrasonography in the lowest quartile group were higher than those in the adequate magnesium group (0.851-1.15 mmol/L) (p < 0.01, odds ratios [ORs] = 2.748-3.236). The risks of total and subclinical-only hypothyroidism in the lowest quartile group were higher than those in the adequate magnesium group (0.851-1.15 mmol/L) (p < 0.01, ORs = 4.482-4.971). Severely low serum magnesium levels are associated with an increased rate of TGAb positivity, HT, and hypothyroidism.

Biofortification of Cereals With Foliar Selenium and Iodine Could Reduce Hypothyroidism

Concurrent selenium and iodine deficiencies are widespread, in both developing and developed countries. Salt iodisation is insufficient to ensure global iodine adequacy, with an estimated one-third of humanity at risk of hypothyroidism and associated iodine deficiency disorders (IDD). Agronomic biofortification of food crops, especially staples such as cereals, which are consumed widely, may be an effective component of a food system strategy to reduce selenium and iodine malnutrition. Iodine and selenium are needed in the optimum intake range for thyroid health, hence joint biofortification makes sense for areas deficient in both. Foliar application is recommended as the most effective, efficient, least wasteful method for selenium and iodine biofortification. Currently, selenium is easier to increase in grain, fruit, and storage roots by this method, being more phloem mobile than iodine. Nevertheless, strategic timing (around heading is usually best), use of surfactants and co-application with potassium nitrate can increase the effectiveness of foliar iodine biofortification. More research is needed on iodine transporters and iodine volatilisation in plants, bioavailability of iodine in biofortified plant products, and roles for nano selenium and iodine in biofortification. For adoption, farmers need an incentive such as access to a premium functional food market, a subsidy or increased grain yield resulting from possible synergies with co-applied fertilisers, enhancers, fungicides, and insecticides. Further research is needed to inform these aspects of foliar agronomic biofortification.

Is selenium supplementation in autoimmune thyroid diseases justified?

PURPOSE OF REVIEW

This review provides an appraisal of recent evidence for or against selenium supplementation in patients with autoimmune thyroid diseases, and discusses possible effect mechanisms.

RECENT FINDINGS

Epidemiological data suggest an increased prevalence of autoimmune thyroid diseases under conditions of low dietary selenium intake. Two systematic reviews have evaluated controlled trials among patients with autoimmune thyroiditis and report that selenium supplementation decreases circulating thyroid autoantibodies. The immunomodulatory effects of selenium might involve reducing proinflammatory cytokine release. However, clinically relevant effects of selenium supplementation, including improvement in quality of life, are more elusive. In Graves' disease, some, but not all, trials indicate that adjuvant selenium supplementation enhances the restoration of biochemical euthyroidism, and might benefit patients with mild Graves' orbitopathy.

SUMMARY

The use of selenium supplementation as adjuvant therapy to standard thyroid medication may be widespread, but a growing body of evidence yields equivocal results. The available evidence from trials does not support routine selenium supplementation in the standard treatment of patients with autoimmune thyroiditis or Graves' disease. However, correction of moderate to severe selenium deficiency may offer benefits in preventing, as well as treating, these disorders. Molecular mechanisms have been proposed, but further studies are needed.

Selenium Status Is Positively Associated with Bone Mineral Density in Healthy Aging European Men

Objective

It is still a matter of debate if subtle changes in selenium (Se) status affect thyroid function tests (TFTs) and bone mineral density (BMD). This is particularly relevant for the elderly, whose nutritional status is more vulnerable.

Design and Methods

We investigated Se status in a cohort of 387 healthy elderly men (median age 77 yrs; inter quartile range 75–80 yrs) in relation to TFTs and BMD. Se status was determined by measuring both plasma selenoprotein P (SePP) and Se.

Results

The overall Se status in our population was low normal with only 0.5% (2/387) of subjects meeting the criteria for Se deficiency. SePP and Se levels were not associated with thyroid stimulating hormone (TSH), free thyroxine (FT4), thyroxine (T4), triiodothyronine (T3) or reverse triiodothyronine (rT3) levels. The T3/T4 and T3/rT3 ratios, reflecting peripheral metabolism of thyroid hormone, were not associated with Se status either. SePP and Se were positively associated with total BMD and femoral trochanter BMD. Se, but not SePP, was positively associated with femoral neck and ward's BMD. Multivariate linear analyses showed that these associations remain statistically significant in a model including TSH, FT4, body mass index, physical performance score, age, smoking, diabetes mellitus and number of medication use.

Conclusion

Our study demonstrates that Se status, within the normal European marginally supplied range, is positively associated with BMD in healthy aging men, independent of thyroid function. Thyroid function tests appear unaffected by Se status in this population.

Association of serum selenium with thyroxin in severely iodine-deficient young children from the Amhara region of Ethiopia

BACKGROUND/OBJECTIVES

Selenium (Se) as part of glutathione peroxidase and iodothyronine deiodinase enzymes influences thyroid metabolism. This study investigated the association of serum Se levels with thyroid metabolism of severely iodine-deficient young children from the Amhara region of Ethiopia.

SUBJECTS/METHODS

In a cross-sectional study, Se, thyroid-stimulating hormone, total thyroxin, total triiodothyronine and thyroglobulin in serum of children (N=628) 54-60 months of age from the Amhara region, Ethiopia, were analyzed. In addition, iodine in urine and household salt was analyzed, and the presence of goiter was assessed.

RESULTS

The median serum Se concentration was 61.4 μg/l (10.7-290.9 μg/l). Selenium deficiency (serum Se <70 μg/l) was detected in 57.8% (N=349) of the children. The median urinary iodine concentration (UIC) was 9.8 μg/l. The majority (86.6%, N=449) of children had UIC below the recommended value (100 μg/l). In addition, 59.8% (N=310) of children were severely iodine deficient (UIC<20 μg/l). Only 12.7% of salt samples had iodine. Goiter was present in 44.6% (N=280) of the children. Selenium-deficient children had higher serum thyroxin (T4) than children with normal serum Se concentration (P<0.001).

CONCLUSIONS

Serum Se was negatively associated with T4 level in young children from the Amhara region of Ethiopia and may endanger the effectiveness of the salt iodization program.

Selenium and the thyroid

PURPOSE OF REVIEW

This article provides an update on the role of the essential trace element selenium and its interaction with the other trace elements iodine and iron that together contribute to adequate thyroid hormone status. Synthesis, secretion, metabolism and action of thyroid hormone in target tissues depend on a balanced nutritional availability or supplementation of these elements. Selenium status is altered in benign and malignant thyroid diseases and various selenium compounds have been used to prevent or treat widespread diseases such as goiter, autoimmune thyroid disease or thyroid cancer.

RECENT FINDINGS

Several studies, most with still too low numbers of cases, indicate that selenium administration in both autoimmune thyroiditis (Hashimoto thyroiditis) and mild Graves' disease improves clinical scores and well-being of patients and reduces thyroperoxidase antibody titers. However, published results are still conflicting depending on basal selenium status, dose, time and form of selenium used for intervention. Evidence for sex-specific selenium action, lack of beneficial effects in pregnancy and contribution of genetic polymorphisms (selenoprotein S) has been presented.

SUMMARY

Adequate nutritional supply of selenium that saturates expression of circulating selenoprotein P, together with optimal iodine and iron intake, is required for a healthy and functional thyroid during development, adolescence, adulthood and aging.

Does selenium supplementation affect thyroid function? Results from a randomized, controlled, double-blinded trial in a Danish population

OBJECTIVE

Selenium is present in the active site of proteins important for thyroid hormone synthesis and metabolism. The objective of this study is to investigate the effect of selenium supplementation in different doses on thyroid function, under conditions of suboptimal dietary selenium intake.

DESIGN

The Danish PREvention of Cancer by Intervention with SElenium pilot study (DK-PRECISE) is a randomized, double-blinded, placebo-controlled trial. A total of 491 males and females aged 60-74 years were randomized to 100 μg (n=124), 200 μg (n=122), or 300 μg (n=119) selenium-enriched yeast or matching yeast-based placebo tablets (n=126). A total of 361 participants, equally distributed across treatment groups, completed the 5-year intervention period.

METHODS

Plasma samples were analyzed for selenium and serum samples for TSH, free triiodothyronine (FT3), and free thyroxine (FT4) at baseline, and after 6 months, and 5 years of supplementation.

RESULTS

Plasma selenium concentrations increased significantly and dose-dependently in treatment groups receiving selenium (P<0.001). Serum TSH and FT4 concentrations decreased significantly and dose-dependently by 0.066 mIU/l (P=0.010) and 0.11 pmol/l (P=0.015), respectively, per 100 μg/day increase, with insignificant differences between 6 months and 5 years. No significant effects were found for FT3 and FT3:FT4 ratio.

CONCLUSIONS

In euthyroid subjects, selenium supplementation minutely and dose-dependently affects thyroid function, when compared with placebo, by decreasing serum TSH and FT4 concentrations. Based on these findings, selenium supplementation is not warranted under conditions of marginal selenium deficiency. However, a role for selenium supplementation in the treatment of autoimmune thyroid diseases is still unresolved.

Selenium Supplementation for Hashimoto's Thyroiditis: Summary of a Cochrane Systematic Review

Selenium supplementation in people with Hashimoto's thyroiditis might reduce antibody levels and result in a decreased dosage of levothyroxine (LT4) and may provide other beneficial effects (e.g. on mood and health-related quality of life). The aim of our systematic review was to assess the effects of selenium supplementation on Hashimoto's thyroiditis. We searched The Cochrane Library, MEDLINE, EMBASE and Web of Science for randomized controlled trials. Study selection, data extraction, assessment of risk of bias and analyses were carried out by two independent review authors. We assessed the quality of the evidence of included studies using GRADE. Four studies rated at unclear to high risk of bias comprising 463 participants were included. One study at high risk of bias showed statistically significant improvement in subjective well-being with sodium selenite 200 μg plus titrated LT4 compared with placebo plus titrated LT4 (RR 4.67, 95% CI 1.61-13.50). Selenomethionine 200 μg as a single treatment or combined with LT4 reduced the serum levels of anti-thyroid peroxidase antibodies compared with placebo (or placebo plus LT4) in three studies (p < 0.001). Although the changes from baseline were statistically significant in these three studies, their clinical relevance is unclear. In conclusion, the results of these four studies, assessed at unclear to high risk of bias, show that evidence to support or refute the efficacy of selenium supplementation in people with Hashimoto's thyroiditis is incomplete and not reliable to help inform clinical decision making.

Selenium and the thyroid

PURPOSE OF REVIEW

To provide information on the role of the essential trace element selenium, which enables appropriate thyroid hormone synthesis, secretion, and metabolism, and to discuss supplementation with various selenium compounds, which prevent thyroid diseases such as goiter and exert beneficial effects in thyroid autoimmune diseases.

RECENT FINDINGS

Selenium administration in both autoimmune thyroiditis (M. Hashimoto) and mild Graves' disease improves clinical scores and well-being of patients and reduces autoimmune antibody titres in several prospective, placebo-controlled supplementation studies.

SUMMARY

Adequate nutritional supply of selenium, together with the two other essential trace elements iodine and iron, is required for a healthy thyroid during development and adolescence, as well as in the adult and aging populations.

Selenium supplementation for Hashimoto's thyroiditis

BACKGROUND

Hashimoto's thyroiditis is a common auto-immune disorder. The most common presenting symptoms may include anxiety, negative mood, depression, dry skin, cold intolerance, puffy eyes, muscle cramps and fatigue, deep voice, constipation, slow thinking and poor memory. Clinical manifestations of the disease are defined primarily by low levels of thyroid hormones; therefore it is treated by hormone replacement therapy, which usually consists of levothyroxine (LT4). Selenium might reduce antibody levels and result in a decreased dosage of LT4 and may provide other beneficial effects (e.g. on mood and health-related quality of life).

OBJECTIVES

To assess the effects of selenium supplementation on Hashimoto's thyroiditis.

SEARCH METHODS

We searched the following databases up to 2 October 2012: CENTRAL in The Cochrane Library (2012, Issue 10), MEDLINE, EMBASE, and Web of Science; we also screened reference lists of included studies and searched several online trial registries for ongoing trials (5 November 2012).

SELECTION CRITERIA

Randomised controlled clinical trials that assessed the effects of selenium supplementation for adults diagnosed with Hashimoto's thyroiditis.

DATA COLLECTION AND ANALYSIS

Study selection, data extraction, assessment of risk of bias, and analyses were carried out by two independent review authors. We assessed the quality of the evidence of included studies using GRADE. We were unable to conduct a meta-analysis because clinical heterogeneity between interventions that were investigated is substantial.

MAIN RESULTS

Four studies at unclear to high risk of bias comprising 463 participants were included. The mean study duration was 7.5 months (range 3 to 18 months). One of our primary outcomes－'change from baseline in health related quality of life'－and two of our secondary outcomes－'change from baseline in LT4 replacement dosage at end of the study' and 'economic costs'－were not assessed in any of the studies. One study at high risk of bias showed statistically significant improvement in subjective well-being with sodium selenite 200 μg plus titrated LT4 compared with placebo plus titrated LT4 (relative risk (RR) 4.67, 95% confidence interval (CI) 1.61 to 13.50; P = 0.004; 36 participants; number needed to treat (NNT) = 2 (95% CI 2 to 3)).Selenomethionine 200 μg reduced the serum levels of anti-thyroid peroxidase antibodies compared with placebo in two studies (mean difference (MD) -917 U/mL, 95% CI -1056 to -778; P < 0.001; 85 participants) and (MD -345 IU/mL, 95% CI -359 to -331; P < 0.001; 169 participants). Pooling of the studies was not feasible due to marked clinical heterogeneity (I(2) = 99%). In a further comparison within the first study where selenomethionine was combined with LT4 the reduction in TPO antibodies was even more noticeable (MD -1508 U/mL, 95% CI -1671 to -1345; P < 0.001; 86 participants). In a third study, where LT4 was added to both intervention arms, a reduction in serum levels of anti-thyroid peroxidase antibodies favoured the selenomethionine arm as well (MD -235 IU/mL, 95% CI -374 to -95; P = 0.001; 88 participants). Although the changes from baseline were statistically significant in these three studies, their clinical relevance is unclear. Serum antibodies were not statistically significantly affected in the study comparing sodium selenite 200 μg plus titrated LT4 with placebo plus titrated LT4 (MD -25, 95% CI -181 to 131; P = 0.75; 36 participants).Adverse events were reported in two studies (1 of 85 and 1 of 88 participants, respectively). Selenium supplementation did not appear to have a statistically significant impact on the incidence of adverse events (RR 2.93, 95% CI 0.12 to 70.00; and RR 2.63, 95% CI 0.11 to 62.95).

AUTHORS' CONCLUSIONS

Results of these four studies show that evidence to support or refute the efficacy of selenium supplementation in people with Hashimoto's thyroiditis is incomplete. The current level of evidence for the efficacy of selenium supplementation in the management of people with Hashimoto's thyroiditis is based on four randomised controlled trials assessed at unclear to high risk of bias; this does not at present allow confident decision making about the use of selenium supplementation for Hashimoto's thyroiditis. This review highlights the need for randomised placebo-controlled trials to evaluate the effects of selenium in people with Hashimoto's thyroiditis and can ultimately provide reliable evidence to help inform clinical decision making.

Interventions for clinical and subclinical hypothyroidism pre-pregnancy and during pregnancy

BACKGROUND

Over the last decade there has been enhanced awareness of the appreciable morbidity of thyroid dysfunction, particularly thyroid deficiency. Since treating clinical and subclinical hypothyroidism may reduce adverse obstetric outcomes, it is crucial to identify which interventions are safe and effective.

OBJECTIVES

To identify interventions used in the management of hypothyroidism and subclinical hypothyroidism pre-pregnancy or during pregnancy and to ascertain the impact of these interventions on important maternal, fetal, neonatal and childhood outcomes.

SEARCH METHODS

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (31 March 2013).

SELECTION CRITERIA

Randomised controlled trials (RCTs) and quasi-randomised controlled trials that compared a pharmacological intervention for hypothyroidism and subclinical hypothyroidism pre-pregnancy or during pregnancy with another intervention or placebo.

DATA COLLECTION AND ANALYSIS

Two review authors assessed trial eligibility and quality and extracted the data.

MAIN RESULTS

We included four RCTs of moderate risk of bias involving 362 women. In one trial of 115 women, levothyroxine therapy to treat pregnant euthyroid (normal thyroid function) women with thyroid peroxidase antibodies was not shown to reduce pre-eclampsia significantly (risk ratio (RR) 0.61; 95% confidence interval (CI) 0.11 to 3.48) but did significantly reduce preterm birth by 72% (RR 0.28; 95% CI 0.10 to 0.80). Two trials of 30 and 48 hypothyroid women respectively compared levothyroxine doses, but both trials reported only biochemical outcomes. A trial of 169 women compared the trace element selenomethionine (selenium) with placebo and no significant differences were seen for either pre-eclampsia (RR 1.44; 95% CI 0.25 to 8.38) or preterm birth (RR 0.96; 95% CI 0.20 to 4.61). None of the four trials reported on childhood neurodevelopmental delay.There was a non-significant trend towards fewer miscarriages with levothyroxine, and selenium showed some favourable impact on postpartum thyroid function and a decreased incidence of moderate to advanced postpartum thyroiditis.

AUTHORS' CONCLUSIONS

This review found no difference between levothyroxine therapy and a control for treating pregnant euthyroid women with thyroid peroxidase antibodies for the outcome of pre-eclampsia, however a reduction in preterm birth and a trend towards reduced miscarriage with levothyroxine was shown. This review also showed no difference for pre-eclampsia or preterm birth when selenium was compared with placebo, however a promising reduction in postpartum thyroiditis was shown. Childhood neurodevelopmental delay was not assessed by any trial included in the review.Given that this review is based on four trials of moderate risk of bias, with only two trials contributing data (n = 284), there is insufficient evidence to recommend the use of one intervention for clinical or subclinical hypothyroidism pre-pregnancy or during pregnancy over another, for improving maternal, fetal, neonatal and childhood outcomes.

Interaction of aqueous Se(IV)/Se(VI) with FeSe/FeSe2: implication to Se redox process

Since reductive precipitation is considered as the most effective way to immobilize (79)Se, interaction of aqueous Se(IV)/Se(VI) with Fe(II)-bearing minerals has received extensive attention. In contrast to the thermodynamic calculations, as well as the prevalence of iron selenide phases observed in soil, sediments and ore deposits, most laboratory experiments have found that Se(0) was the reaction product. In this study, the interaction of Se(IV)/Se(VI) with FeSe/FeSe2 were investigated. The results demonstrate that FeSe and FeSe2 can be oxidized to Se(0) by Se(IV) with relatively fast kinetics, while reaction between Se(VI) and FeSe/FeSe2 only occurs under limited conditions (i.e. in the presence of high ferrous content and higher pH) with much slower kinetics, and there is no evident reaction in most case. Therefore, reduction of Se(IV) by Fe(II)-bearing minerals, in particular by natural occurring minerals, is envisioned to produce Se(0) at the early stage of experiments, rather than FeSe or FeSe2. Due to the formation of bulk Se(0) and its low solubility, the Fe-Se-O-H2O system will maintain redox disequilibrium in laboratory time-scale. This study also reveals that iron selenides, like iron sulfides, have strong reactivity toward Fe(3+). The findings in this study give insight into possible controls on Se redox process.

Selenium and the thyroid gland: more good news for clinicians

The thyroid is the organ with the highest selenium content per gram of tissue because it expresses specific selenoproteins. Since the discovery of myxoedematous cretinism and thyroid destruction following selenium repletion in iodine- and selenium-deficient children, data on links between thyroid metabolism and selenium have multiplied. Although very minor amounts of selenium appear sufficient for adequate activity of deiodinases, thus limiting the impact of its potential deficiency on synthesis of thyroid hormones, selenium status appears to have an impact on the development of thyroid pathologies. The value of selenium supplementation in autoimmune thyroid disorders has been emphasized. Most authors attribute the effect of supplementation on the immune system to the regulation of the production of reactive oxygen species and their metabolites. In patients with Hashimoto's disease and in pregnant women with anti-TPO antibodies, selenium supplementation decreases anti-thyroid antibody levels and improves the ultrasound structure of the thyroid gland. Although clinical applications still need to be defined for Hashimoto's disease, they are very interesting for pregnant women given that supplementation significantly decreases the percentage of postpartum thyroiditis and definitive hypothyroidism. In Graves' disease, selenium supplementation results in euthyroidism being achieved more rapidly and appears to have a beneficial effect on mild inflammatory orbitopathy. A risk of diabetes has been reported following long-term selenium supplementation, but few data are available on the side effects associated with such supplementation and further studies are required.

Selenium and the thyroid: A close-knit connection

INTRODUCTION

In areas with severe selenium deficiency higher incidence of thyroiditis has been reported due to a decreased activity of selenium-dependent glutathione peroxidase enzyme within thyroid cells.

AIMS AND OBJECTIVE

To study the effect of selenium supplementation in patients with autoimmune thyroid disease.

MATERIALS AND METHODS

This is a blinded placebo-controlled prospective study done in 60 patients with autoimmune thyroid disease (as defined by an anti-thyroid peroxidase antibody (TPOAb) level more than 150 IU/ml) irrespective of the baseline thyroid status. Patients with overt hyperthyroidism who are on antithyroid drugs, patients on any other medication, which may alter the immunity status of the patients, and pregnant patients were excluded from the study. Patients were randomized into two age and TPOAb-matched groups; 30 patients received 200 μg of sodium selenite/day, orally, for 3 months, and 30 patients received placebo. All hypothyroid patients were given l-thyroxine replacement.

RESULTS

Of 30 patients in the selenium treated group, 6 patients were overtly hypothyroid, 15 were subclinical hypothyroid, 6 were euthyroid, and 3 were subclinical hyperthyroid. The mean TPOAb concentration decreased significantly by 49.5% (P < 0.013) in the selenium treated group versus 10.1% (P < 0.95) in the placebo-treated group.

CONCLUSION

Selenium substitution has a significant impact on inflammatory activity in thyroid-specific autoimmune disease. It would be of interest to determine whether early treatment with selenium in patients with newly developed autoimmune thyroiditis may delay or even prevent the natural course of these diseases.

Effects of selenium supplementation on the natural course of autoimmune thyroiditis: a short review

Selenium (Se) is an important element that exerts its effects on the selenoproteins. It is an essential component of the glutathione peroxidase enzymes, which have anti-oxidant and anti-inflammatory properties, and a component of iodothyronine selenodeiodinases, which catalyze the extrathyroid production of T3 from T4. Se is important to several aspects of thyroid homeostasis and may influence the natural course of thyroid diseases such as autoimmune thyroiditis (AIT). This review analyzes the effects of Se supplementation in patients with AIT, based on the studies published on this issue to date.

Selenomethionine treatment in patients with autoimmune thyroiditis: a prospective, quasi-randomised trial

AIMS

To test whether selenium administration affects autoantibodies to thyroid peroxidase (anti-TPO) and thyroglobulin (anti-TG) titres in chronic autoimmune (Hashimoto's - HT) thyroiditis.

METHODS

A prospective, open-label, quasi-randomised study in 86 HT patients (n = 86) assigned to either selenomethionine (Seme) 200μg daily for 3 months (Se3, n = 15) or 6 months (Se6, n = 46) or placebo (Control, n = 25). Serum Se, anti-TPO, anti-TG and thyroid hormones were measured in all patients at baseline, 3 and 6 months. A subgroup of 18 patients (twelve on Se6 and six controls) were subjected in thyroid fine-needle biopsy at baseline and 6 months to detect changes in lymphocyte infiltration.

RESULTS

No significant difference in anti-TPO levels was recorded after 3 (p = 0.88) or 6 months (p = 0.62) on Seme. Anti-TG levels decreased both at 3 months (p = 0.001) and 6 months (p = 0.001). No significant changes in thyroid stimulating hormone, free thyroxine and free triiodothyronine levels or in the lymphocytes' number in thyroid cytology specimens were detected. Age, gender, duration of disease, baseline anti-TPO levels and per cent change in Se levels could not predict the response of anti-TPO levels to Seme administration.

CONCLUSION

Our data suggest that Seme administration in pharmacological doses for a period of 6 months seems to have no significant effect on serum thyroid auto-antibodies' levels or lymphocyte infiltration of the thyroid gland.

Environmental selenium research: from microscopic processes to global understanding

Selenium is a natural trace element that is of fundamental importance to human health. The extreme geographical variation in selenium concentrations in soils and food crops has resulted in significant health problems related to deficient or excess levels of selenium in the environment. To deal with these kinds of problems in the future it is essential to get a better understanding of the processes that control the global distribution of selenium. The recent development of analytical techniques and methods enables accurate selenium measurements of environmental concentrations, which will lead to a better understanding of biogeochemical processes. This improved understanding may enable us to predict the distribution of selenium in areas where this is currently unknown. These predictions are essential to prevent future Se health hazards in a world that is increasingly affected by human activities.

Postpartum thyroiditis: an autoimmune thyroid disorder which predicts future thyroid health

Postpartum thyroiditis is a potentially destructive lymphocytic thyroiditis occurring in approximately 8% of the pregnant population, making it the most common endocrine disorder associated with pregnancy. This autoimmune thyroid disorder is precipitated by the postpartum immunological rebound that follows the partial immunosuppression of pregnancy, in individuals already at risk of autoimmune thyroid disease. The manifestations of postpartum thyroiditis are usually not present at the six-week postpartum visit and thus it is important that all physicians be aware of the risks, presentation and intervention required for this common disorder. Postpartum thyroiditis is a strong predictor of future thyroid health and it is essential that women with a history of postpartum thyroiditis be screened regularly, especially prior to a future pregnancy. Selenium supplementation has recently been identified as a potential means to prevent postpartum thyroiditis in women at risk but further studies are required before recommendations for its use can be made.

Interventions for clinical and subclinical hypothyroidism in pregnancy

BACKGROUND

Over the last decade there has been enhanced awareness of the appreciable morbidity of thyroid dysfunction, particularly thyroid deficiency. Since treating clinical and subclinical hypothyroidism may reduce adverse obstetric outcomes, it is crucial to identify which interventions are safe and effective.

OBJECTIVES

To identify interventions used in the management of hypothyroidism and subclinical hypothyroidism in pregnancy and to ascertain the impact of these interventions on important maternal, fetal, neonatal and childhood outcomes.

SEARCH STRATEGY

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (November 2009).

SELECTION CRITERIA

Randomised controlled trials (RCTs) that compared a pharmacological intervention for hypothyroidism and subclinical hypothyroidism in pregnancy with another intervention or placebo.

DATA COLLECTION AND ANALYSIS

Two review authors assessed trial eligibility and quality and extracted the data.

MAIN RESULTS

We included three RCTs of moderate risk of bias involving 314 women. In one trial of 115 women, levothyroxine therapy to treat pregnant euthyroid women with thyroid peroxidase antibodies was not shown to reduce pre-eclampsia significantly (risk ratio (RR) 0.61; 95% confidence interval (CI) 0.11 to 3.48) but did significantly reduce preterm birth by 72% (RR 0.28; 95% CI 0.10 to 0.80). One trial of 30 hypothyroid women compared levothyroxine doses, but only reported biochemical outcomes. A trial of 169 women compared the trace element selenomethionine (selenium) with placebo and no significant differences were seen for either pre-eclampsia (RR 1.44; 95% CI 0.25 to 8.38) or preterm birth (RR 0.96; 95% CI 0.20 to 4.61). None of the three trials reported on childhood neurodevelopmental delay.There was a non-significant trend towards fewer miscarriages with levothyroxine, and selenium showed some favourable impact on postpartum thyroid function and decreased incidence of moderate to advanced postpartum thyroiditis.

AUTHORS' CONCLUSIONS

Levothyroxine treatment of clinical hypothyroidism in pregnancy is already standard practice given the documented benefits from earlier non-randomised studies. Whether levothyroxine should be utilised in autoimmune and subclinical hypothyroidism remains to be seen, but it may prove worthwhile, given a possible reduction in preterm birth and miscarriage.Selenomethionine as an intervention in women with thyroid autoantibodies is promising, particularly in reducing postpartum thyroiditis. There is a probable low incidence of adverse outcomes from levothyroxine and selenomethionine. High-quality evidence is lacking and large-scale randomised trials are urgently needed in this area. Until evidence for or against universal screening becomes available, targeted thyroid function testing in pregnancy should be implemented in women at risk of thyroid disease and levothyroxine utilised in hypothyroid women.

The impact of common micronutrient deficiencies on iodine and thyroid metabolism: the evidence from human studies

Deficiencies of micronutrients are highly prevalent in low-income countries. Inadequate intake of iodine impairs thyroid function and results in a spectrum of disorders. Other common deficiencies of micronutrients such as iron, selenium, vitamin A, and possibly zinc may interact with iodine nutrition and thyroid function. Randomised controlled intervention trials in iodine- and iron-deficient populations have shown that providing iron along with iodine results in greater improvements in thyroid function and volume than providing iodine alone. Vitamin A supplementation given alone or in combination with iodised salt can have a beneficial impact on thyroid function and thyroid size. Despite numerous studies of the effect of selenium on iodine and thyroid metabolism in animals, most published randomised controlled intervention trials in human populations failed to confirm an impact of selenium supplementation on thyroid metabolism. Little evidence is available on interactions between iodine and zinc metabolism.

Selenium and thyroid

Inadequate supply of the essential trace element selenium (Se) has been associated with predisposition for, or manifestation of, various human diseases such as Keshan and Kashin-Beck disease, cancer, impaired immune function, neurodegenerative and age-related disorders and disturbances of the thyroid hormone axis. Se deficiency in combination with inadequate iodine contributes to the pathogenesis of myxedematous cretinism. The recent identification of various distinct selenocysteine-containing proteins, encoded by 25 human genes, provides information on the molecular and biochemical basis of beneficial and possible adverse effects of this trace element. The thyroid gland is among the human tissues with the highest Se content per mass unit similar to other endocrine organs and the brain. Selenoproteins involved in cellular antioxidative defence systems and redox control, such as the glutathione peroxidase (GPx) and the thioredoxin reductase (TxnRd) family, are involved in protection of the thyroid gland from excess hydrogen peroxide and reactive oxygen species produced by the follicles for biosynthesis of thyroid hormones. In addition, the three key enzymes involved in activation and inactivation of thyroid hormones, the iodothyronine deiodinases (DIO1,2,3), are selenoproteins with development, cell- and pathology-related expression patterns. While nutritional Se supply is normally sufficient for adequate expression of functional Dio enzymes with exception of long-term parenteral nutrition and certain diseases impairing gastrointestinal absorption of Se compounds, the nutritional Se supply for the protection of the thyroid gland and synthesis of some more abundant selenoproteins of the GPx and the TrxR family might be limiting their proper expression under (patho-)physiological conditions.

Selenium and iodine supplementation: effect on thyroid function of older New Zealanders

BACKGROUND

The New Zealand population has both marginal selenium status and mild iodine deficiency. Adequate intakes of iodine and selenium are required for optimal thyroid function.

OBJECTIVE

The aim of the study was to determine whether low selenium and iodine status compromises thyroid function in an older New Zealand population.

DESIGN

We investigated the effects of selenium and iodine supplementation in a double-blind, randomized, placebo-controlled trial in 100 Dunedin volunteers aged 60-80 y. Participants received 100 microg Se/d as l-selenomethionine, 80 microg I, 100 microg Se + 80 microg I, or placebo for 3 mo. Thyroid-stimulating hormone (TSH), free triiodothyronine (T(3)), free thyroxine (T(4)), thyroglobulin, plasma selenium, whole-blood glutathione peroxidase (GPx) activity, and urinary iodine concentrations (UICs) were measured.

RESULTS

Plasma selenium (P < 0.0001) and whole-blood GPx activity (P<0.0001) increased from baseline to week 12 in the selenium and selenium plus iodine groups in comparison with the placebo group. Median UIC at baseline was 48 microg/L (interquartile range: 31-79 microg/L), which is indicative of moderate iodine deficiency. UIC increased in the iodine and selenium plus iodine groups and was significant only for the iodine group (P = 0.0014). Thyroglobulin concentration decreased by 24% and 13% of baseline in the iodine and selenium plus iodine groups in comparison with the placebo group (P = 0.009 and P = 0.108, respectively). No significant treatment effects were found for TSH, free T(3), free T(4), or ratio of T(3) to T(4).

CONCLUSIONS

Additional selenium improved GPx activity but not the thyroid hormone status of older New Zealanders. Iodine supplementation alleviated the moderate iodine deficiency and reduced elevated thyroglobulin concentrations. No synergistic action of selenium and iodine was observed. The trial was registered at www.anzctr.org.au/registry/ as ACTRN012605000368639.

Effects of selenomethionine supplementation on selenium status and thyroid hormone concentrations in healthy adults

BACKGROUND

Selenium, a potential cancer prevention agent currently being tested against prostate cancer in the Selenium and Vitamin E Cancer Prevention Trial (SELECT), plays an integral role in thyroid metabolism. The effects of long-term selenium supplementation on thyroid hormone concentrations are unknown.

OBJECTIVE

The objective was to investigate the effects of long-term selenium supplementation on thyroid hormone concentrations.

DESIGN

Twenty-eight healthy adults took 200 microg selenomethionine/d for 28 mo. The thyroid hormones triiodothyronine (T3), thyroxine (T4), and thyrotropin (TSH) were measured in plasma for 4 mo before supplementation and quarterly during supplementation. The assay methods were changed midstudy; the results of the 2 methods were not comparable. Therefore, one analysis was conducted based on the results of the first method, and a second analysis was based on all of the data, adjusted for the change. Serial data collection permitted a test for trends rather than simply a difference between initial and final values.

RESULTS

By 9 mo, mean (+/-SEM) plasma selenium concentrations had increased from 1.78 +/- 0.07 micromol/L at baseline to 2.85 +/- 0.11 micromol/L for men and from 1.64 +/- 0.04 to 3.32 +/- 0.1.2 micromol/L for women. T3 concentrations in men increased 5% per year (P = 0.01). T4 and TSH concentrations were unchanged.

CONCLUSIONS

Selenium supplementation produced no clinically significant changes in thyroid hormone concentrations. A small but statistically significant increase in T3 concentrations was noted in men, with no corresponding decreases in TSH. A subset of SELECT subjects might be monitored periodically for changes during long-term selenium supplementation.

Selenium deficiency as a possible contributor of goiter in schoolchildren of Isfahan, Iran

The prevalence of goiter still remains high in some areas of Iran in spite of iodine supplementation. In the present study, we investigated the role of selenium (Se) deficiency in the etiology of goiter in Isfahan. Two thousand three hundred thirty-one schoolchildren were selected by multistage random sampling. Thyroid size was estimated in each child by inspection and palpation. Urinary iodine concentration (UIC) and plasma Se were measured. Overall, 32.9% of the 2,331 children had goiter. The median UIC was 19.55 microg/dl. Plasma Se was measured in 96 goitrous and 72 nongoitrous children. The mean +/- SD of plasma Se in goitrous and nongoitrous children was 66.86 +/- 21.82 and 76.67 +/- 23.33 microg/l, respectively (P = 0.006). Goitrous girls had lower plasma Se level than nongoitrous girls (65.62 +/- 21.64 vs. 76.51 +/- 22.61 microg/dl, P = 0.02). Goitrous boys had lower plasma Se level than nongoitrous boys (68.45 +/- 22.21 vs. 76.91 +/- 24.76 microg/l, P = 0.14). The prevalence of Se deficiency was significantly higher in goitrous boys and girls than nongoitrous children. Se deficiency is among the contributors of goiter in Isfahan goitrous schoolchildren. However, the role of other micronutrient deficiencies or goitrogens should be investigated in this region.

The interactions between selenium and iodine deficiencies in man and animals

Up to one billion people live in areas where they may be at risk from I deficiency. Many of the debilitating effects of the deficiency may be irreversible, consequently it is essential to understand the mechanisms whereby lack of I can cause disease through decreased thyroxine and 3, 3',5-triiodothyronine (T3) synthesis. Since Se has an essential role in thyroid hormone metabolism, it has the potential to play a major part in the outcome of I deficiency. These effects of Se derive from two aspects of its biological function. First, three Se-containing deiodinases regulate the synthesis and degradation of the biologically active thyroid hormone, T3. Second, selenoperoxidases and possibly thioredoxin reductase (EC1.6.4.5) protect the thyroid gland from H2O2produced during the synthesis of thyroid hormones. The mechanisms whereby Se deficiency exacerbates the hypothyroidism due to I deficiency have been elucidated in animals. In contrast to these adverse effects, concurrent Se deficiency may also cause changes in deiodinase activities which can protect the brain from low T3concentrations in I deficiency. Animals with Se and I deficiency have changes in serum thyroid hormone concentrations that are similar to those observed in patients with I deficiency disease. However such animal models show no thyroid involution, a feature which is characteristic of myxoedematous cretinism in man. These observations imply that if Se deficiency is involved in the outcome of I deficiency in human populations it is likely that other interacting factors such as goitrogens are also implicated. Nevertheless the protection of the thyroid gland from H2O2and the regulation of tissue T3levels are the functions of Se that are most likely to underlie the interactions of Se and I.

Randomized controlled trial of the effect of selenium supplementation on thyroid function in the elderly in the United Kingdom

BACKGROUND

Thyroid function depends on the essential trace mineral selenium, which is at the active center of the iodothyronine deiodinase enzymes that catalyze the conversion of the prohormone thyroxine (T(4)) to the active form of thyroid hormone, triiodothyronine (T(3)).

OBJECTIVE

Because selenium intake in the United Kingdom has fallen during the past 25 y, we wanted to determine whether current selenium status might be limiting conversion of T(4) to T(3) in the elderly, in whom marginal hypothyroidism is relatively common.

DESIGN

We investigated the effect of selenium supplementation in a double-blind, placebo-controlled trial in 501 elderly UK volunteers. Similar numbers of men and women from each of 3 age groups, 60-64 y, 65-69 y, and 70-74 y, were randomly allocated to receive 100, 200, or 300 microg Se/d as high-selenium yeast or placebo yeast for 6 mo. As part of the study, plasma selenium, thyroid-stimulating hormone, and total and free T(3) and T(4) were measured. Data from 368 euthyroid volunteers who provided blood samples at baseline and 6 mo were analyzed.

RESULTS

Although selenium status at baseline correlated weakly with free T(4) (r = -0.19, P < 0.001) and with the ratio of free T(3) to free T(4) (r = 0.12, P = 0.02), we found no evidence of any effect of selenium supplementation on thyroid function, despite significant increases in plasma selenium. However, baseline plasma selenium in our study (x: 91 microg/L) was somewhat higher than in previous supplementation studies in which apparently beneficial effects were seen.

CONCLUSION

We found no indication for increasing selenium intake to benefit T(4) to T(3) conversion in the elderly UK population.

The influence of selenium supplementation on postpartum thyroid status in pregnant women with thyroid peroxidase autoantibodies

CONTEXT

Pregnant women who are positive for thyroid peroxidase antibodies [TPOAb(+)] are prone to develop postpartum thyroid dysfunction (PPTD) and permanent hypothyroidism. Selenium (Se) decreases thyroid inflammatory activity in patients with autoimmune thyroiditis.

OBJECTIVE

We examined whether Se supplementation, during and after pregnancy, influences the thyroidal autoimmune pattern and function.

DESIGN

This was a prospective, randomized, placebo-controlled study.

SETTING

The study was conducted in the Department of Obstetrics and Gynecology and Department of Endocrinology.

PATIENTS

A total of 2143 euthyroid pregnant women participated in the study; 7.9% were TPOAb(+).

INTERVENTIONS

During pregnancy and the postpartum period, 77 TPOAb(+) women received selenomethionine 200 microg/d (group S1), 74 TPOAb(+) women received placebo (group S0), and 81 TPOAb(-) age-matched women were the control group (group C).

MAIN OUTCOME MEASURES

We measured the prevalence of PPTD and hypothyroidism.

RESULTS

PPTD and permanent hypothyroidism were significantly lower in group S1 compared with S0 (28.6 vs. 48.6%, P<0.01; and 11.7 vs. 20.3%, P<0.01).

CONCLUSION

Se supplementation during pregnancy and in the postpartum period reduced thyroid inflammatory activity and the incidence of hypothyroidism.

Effects of selenium depletion and selenium repletion by choice feeding on selenium status of young and old laying hens

We investigated the effect of choice feeding two diets with different selenium (Se) content to young and old moderately Se-deficient laying hens on serum Se (SSe), glutathione peroxidase (GPX), vitamin E, creatine kinase (CK), aspartate aminotransferase (ASAT), thyroxine (T4) and triiodothyronine (T3). Each of two consecutive study parts (I and II) with the same hens and treatments began with a 6-week baseline period (Medium-Se diet), followed by a 9-week depletion period (Low-Se or Medium-Se diet), followed by a 6-week choice period with two different diets offered simultaneously (Medium-Se/Low-Se, Medium-Se/High-Se, or Low-Se/High-Se). During both depletion periods, SSe and GPX gradually decreased, whereas T4 gradually increased in hens fed Low-Se confirming gradual Se-depletion. T3 decreased transiently in young hens only. As reported earlier, Se-deficient hens preferred High-Se over Low-Se diet during the first 3 weeks of choice feeding in part I, not however in part II. This preference resulted in higher SSe in these hens. GPX activity did not reflect feed preference, probably because Se-intake exceeded Se-requirement for maximal GPX activity. In Part II, hens depleted with Low-Se diet had higher SSe when previously offered High-Se diet in either combination, than when offered Low-Se/Medium-Se, presumably due to Se-stores built during choice feeding in part I, which possibly prevented development of Se-deficiency in part II. In addition, in older hens, Se depletion proceeded faster, whereas Se-repletion by choice feeding was slower than in young hens, indicating the increase in Se requirement with advancing age. Vitamin E, ASAT and CK remained largely unchanged by the treatments.

Selenium, the thyroid, and the endocrine system

Recent identification of new selenocysteine-containing proteins has revealed relationships between the two trace elements selenium (Se) and iodine and the hormone network. Several selenoproteins participate in the protection of thyrocytes from damage by H(2)O(2) produced for thyroid hormone biosynthesis. Iodothyronine deiodinases are selenoproteins contributing to systemic or local thyroid hormone homeostasis. The Se content in endocrine tissues (thyroid, adrenals, pituitary, testes, ovary) is higher than in many other organs. Nutritional Se depletion results in retention, whereas Se repletion is followed by a rapid accumulation of Se in endocrine tissues, reproductive organs, and the brain. Selenoproteins such as thioredoxin reductases constitute the link between the Se metabolism and the regulation of transcription by redox sensitive ligand-modulated nuclear hormone receptors. Hormones and growth factors regulate the expression of selenoproteins and, conversely, Se supply modulates hormone actions. Selenoproteins are involved in bone metabolism as well as functions of the endocrine pancreas and adrenal glands. Furthermore, spermatogenesis depends on adequate Se supply, whereas Se excess may impair ovarian function. Comparative analysis of the genomes of several life forms reveals that higher mammals contain a limited number of identical genes encoding newly detected selenocysteine-containing proteins.

Regulation of type 1 iodothyronine deiodinase in health and disease

The major physiologic function of type 1 iodothyronine deiodinase (D1) is to produce triiodothyronine (T(3)) for the plasma. D1 activity is regulated by numerous factors, perhaps the most important of which in human pathophysiology is T(3). T(3) induces D1 expression, contributing to the T(3) excess commonly found in hyperthyroidism. Cytokines, nutritional status, sex steroids, and other factors also regulate D1 activity, although different organs often show different responses. Numerous homeostatic mechanisms can counterbalance isolated changes in D1 expression, such as the genetically decreased expression in C3H/He mice. Two relatively commonly used drugs, propylthiouracil and amiodarone, inhibit D1, which can have substantial effects on circulating thyroid hormone levels. Overall, many factors interact in complex ways to establish D1 levels, contributing to the circulating concentrations of thyroxine (T(4)) and T(3).

Selenium and the control of thyroid hormone metabolism

Thyroid hormone synthesis, metabolism and action require adequate availability of the essential trace elements iodine and selenium, which affect homeostasis of thyroid hormone-dependent metabolic pathways. The three selenocysteine-containing iodothyronine deiodinases constitute a novel gene family. Selenium is retained and deiodinase expression is maintained at almost normal levels in the thyroid gland, the brain and several other endocrine tissues during selenium deficiency, thus guaranteeing adequate local and systemic levels of the active thyroid hormone T(3). Due to their low tissue concentrations and their mRNA SECIS elements deiodinases rank high in the cellular and tissue-specific hierarchy of selenium distribution among various selenoproteins. While systemic selenium status and expression of abundant selenoproteins (glutathione peroxidase or selenoprotein P) is already impaired in patients with cancer, disturbed gastrointestinal resorption, unbalanced nutrition or patients requiring intensive care treatment, selenium-dependent deiodinase function might still be adequate. However, disease-associated alterations in proinflammatory cytokines, growth factors, hormones and pharmaceuticals modulate deiodinase isoenzyme expression independent from altered selenium status and might thus pretend causal relationships between systemic selenium status and altered thyroid hormone metabolism. Limited or inadequate supply of both trace elements, iodine and selenium, leads to complex rearrangements of thyroid hormone metabolism enabling adaptation to unfavorable conditions.

Selenium and iodine supplementation of rural Tibetan children affected by Kashin-Beck osteoarthropathy

BACKGROUND

Kashin-Beck disease is an osteoarthropathy endemic in selenium- and iodine-deficient areas around Lhasa, Tibet.

OBJECTIVE

We assessed the efficacy of selenium supplementation on disease progression.

DESIGN

A double-blind, randomized controlled trial of selenium supplementation was carried out in 324 children aged 5-15 y who had Kashin-Beck disease. Two hundred eighty children received iodized oil before being randomly assigned to receive selenium or placebo, and a control group of 44 subjects was not supplemented at all. Clinical and radiologic signs, selenium status, urinary iodine, and thyroid function were evaluated at baseline and at 12 mo.

RESULTS

The frequencies of joint pain, decreased joint mobility, and radiologic abnormalities were not significantly different between the 3 groups at 12 mo. Height-for-age z scores increased significantly in the subjects who received placebo and iodine or selenium and iodine. In contrast, unsupplemented control subjects did not recover from growth retardation. Serum selenium concentrations at 12 mo were within the reference range and were significantly greater in the selenium-iodine group than in the placebo-iodine group. Serum thyroid hormone concentrations were within the reference ranges after the administration of iodine, and these values were not significantly affected by selenium supplementation.

CONCLUSIONS

The results of this study do not rule out the possibility that selenium may help to prevent the occurrence of Kashin-Beck disease. However, selenium supplementation had no effect on established Kashin-Beck disease, growth, or thyroid function once iodine deficiency was corrected. These results suggest that iodine, but not selenium, deficiency should be corrected in Tibetan children with Kashin-Beck disease.