Goiter and other iodine deficiency disorders: A systematic review of epidemiological studies to deconstruct the complex web

[Normal values for adult thyroid volume on ultrasound - where do they come from and how should they be evaluated?]

The upper limit for the thyroid volume is 18 ml for women and 25 ml for men. These reference values are used and taught in many clinics and practices across several disciplines. The thyroid volume is a relevant factor for the treatment course. But where do the standard values for thyroid volumes come from, what kind of studies have been carried out to establish them? Should these upper limits be used as an universal standard in clinical routine, and which factors can influence the physiological organ size? The following article is dedicated to a critical review of the commonly used reference values for the thyroid volume of adults.

A review of species differences in the control of, and response to, chemical-induced thyroid hormone perturbations leading to thyroid cancer

This review summarises the current state of knowledge regarding the physiology and control of production of thyroid hormones, the effects of chemicals in perturbing their synthesis and release that result in thyroid cancer. It does not consider the potential neurodevelopmental consequences of low thyroid hormones. There are a number of known molecular initiating events (MIEs) that affect thyroid hormone synthesis in mammals and many chemicals are able to activate multiple MIEs simultaneously. AOP analysis of chemical-induced thyroid cancer in rodents has defined the key events that predispose to the development of rodent cancer and many of these will operate in humans under appropriate conditions, if they were exposed to high enough concentrations of the affecting chemicals. There are conditions however that, at the very least, would indicate significant quantitative differences in the sensitivity of humans to these effects, with rodents being considerably more sensitive to thyroid effects by virtue of differences in the biology, transport and control of thyroid hormones in these species as opposed to humans where turnover is appreciably lower and where serum transport of T4/T3 is different to that operating in rodents. There is heated debate around claimed qualitative differences between the rodent and human thyroid physiology, and significant reservations, both scientific and regulatory, still exist in terms of the potential neurodevelopmental consequences of low thyroid hormone levels at critical windows of time. In contrast, the situation for the chemical induction of thyroid cancer, through effects on thyroid hormone production and release, is less ambiguous with both theoretical, and actual data, showing clear dose-related thresholds for the key events predisposing to chemically induced thyroid cancer in rodents. In addition, qualitative differences in transport, and quantitative differences in half life, catabolism and turnover of thyroid hormones, exist that would not operate under normal situations in humans.

Use of Iodine to Biofortify and Promote Growth and Stress Tolerance in Crops

Iodine is not considered essential for land plants; however, in some aquatic plants, iodine plays a critical role in antioxidant metabolism. In humans, iodine is essential for the metabolism of the thyroid and for the development of cognitive abilities, and it is associated with lower risks of developing certain types of cancer. Therefore, great efforts are made to ensure the proper intake of iodine to the population, for example, the iodization of table salt. In the same way, as an alternative, the use of different iodine fertilization techniques to biofortify crops is considered an adequate iodine supply method. Hence, biofortification with iodine is an active area of research, with highly relevant results. The agricultural application of iodine to enhance growth, environmental adaptation, and stress tolerance in plants has not been well explored, although it may lead to the increased use of this element in agricultural practice and thus contribute to the biofortification of crops. This review systematically presents the results published on the application of iodine in agriculture, considering different environmental conditions and farming systems in various species and varying concentrations of the element, its chemical forms, and its application method. Some studies report beneficial effects of iodine, including better growth, and changes in the tolerance to stress and antioxidant capacity, while other studies report that the applications of iodine cause no response or even have adverse effects. We suggested different assumptions that attempt to explain these conflicting results, considering the possible interaction of iodine with other trace elements, as well as the different physicochemical and biogeochemical conditions that give rise to the distinct availability and the volatilization of the element.

Effects of dietary supplementation of selenium and iodine on growth performance, carcass characteristics and histology of thyroid gland in goats

This study assessed the effects of dietary selenium (Se), iodine (I) and a combination of both on growth performance, thyroid gland activity, carcass characteristics and the concentration of iodine and selenium in Longissimus lumborum (LL) muscle in goats. Twenty-four bucks were randomly assigned to four dietary treatments: control (CON), basal diet without supplementation, basal diet + 0.6 mg Se/kg dry matter (DM) (SS), 0.6 mg I/kg DM (IP), or combination of 0.6 mg/kg DM Se and 0.6 mg/kg DM I (SSIP) and fed for 100 days. Animals fed diet SSIP exhibited higher (P < 0.05) body weight and better feed conversion ratio (FCR) than those fed other diets. Dressing percentage of goats fed the supplemented diets was higher (P < 0.05) than that of the control. Carcasses from the IP group had higher (P < 0.05) total fat proportion than the SSIP group. The levels of both elements were significantly elevated (P < 0.05) in LL muscle in supplemented goats. Thyroid follicular epithelial cells of IP and SSIP animals were significantly higher than those of CON and SS groups. The study demonstrated that the combined Se and I dietary supplementation improves growth performance, carcass dressing percentage and increases the retention of Se and I in goat meat.

Does iodine excess lead to hypothyroidism? Evidence from a case-control study in India

BACKGROUND

Iodine deficiency disorders have been known to mankind since antiquity and various researchers elucidated the role of iodine in its causation. However, recent evidence shows that the entire control program ignored multi-causality and association of increased iodine intake with hypothyroidism. This study was conducted to assess differences of iodine intake as measured by urinary iodine excretion (UIE) between cases of hypothyroidism and healthy controls.

METHODS

A case-control study was conducted with three groups (cases, hospital controls and community controls) in two cities of India. Patients with overt hypothyroidism were cases (n = 150) and were compared with age, sex and socioeconomic status-matched hospital (n = 154) and community (n = 488) controls. Thyroid function tests (T3, T4, TSH) were used as diagnostic and inclusion criteria. TPOAb and UIE estimation were carried out for all study participants.

RESULTS

Mean values of TPOAb and UIE were higher in cases as compared to hospital controls as well as community controls (p <0.05). With a cut off of 34 IU/mL for TPOAb, more cases had an anti-TPO level >34 as compared to hospital controls (p <0.001) as well as community controls (p <0.001); OR, 0.06 (95% CI, 0.03, 0.12) and 0.08 (0.05, 0.12), respectively. For UIE cut-off of 300 μg/L, more cases than hospital controls (p = 0.090) and community controls (p = 0.001) had higher levels; OR, 0.671, (0.422, 1.066) and 0.509, (0.348, 0.744), respectively.

CONCLUSION

The study has clearly shown that cases of hypothyroidism are associated with excess iodine intake. Cohort studies to generate further evidence and an eco-social epidemiological approach have been suggested as the way forward.

Acute respiratory failure in a rapidly enlarging benign cervical goitre

Benign goitres have the potential to reach massive sizes if neglected, but most have a protracted course that may or may not present with compressive symptoms. We report the case of a 57-year-old man who presented with a rapidly enlarging nodular goitre resulting in acute respiratory failure. Endotracheal intubation and emergency total thyroidectomy were performed, revealing massive thyroid nodules with minimal intrathoracic extension and tracheal erosion. Despite a course and clinical findings suggestive of malignant disease, histopathology was consistent with a benign multinodular goitre. Several cases of benign goitres necessitating endotracheal intubation have been reported. Airway compromise was attributed to a significant intrathoracic component, or inciting events such as thyroid haemorrhage, pregnancy, radioiodine uptake or major surgery. Obstructive symptoms may not correlate well with objective measures of upper airway obstruction such as radiographs or flow volume loops.

The effect of iodine from iodine-enriched alga Chlorella spp. on the pork iodine content and meat quality in finisher pigs

The aim of the study was to evaluate the effects of iodine from iodine-enriched alga Chlorella spp. on the iodine content in pork and on meat quality traits in finisher pigs. Experimental feed mixtures were fed for a period of 3 months before slaughter. In group A (n = 12), the experimental feed mixture was supplemented with 2 mg I/kg in the form of iodine from iodine-enriched freshwater alga Chlorella spp. The experimental feed mixture for pigs in group B (n = 12) was supplemented with 2 mg I/kg in the form of KI. Group C (n = 12) was the control group without supplementation. Compared to KI, the use of the algal iodine resulted in significantly higher (P < 0.05) iodine concentrations in the thyroid, serum and muscle tissue. No differences between the two groups were found in the meat quality traits. It is concluded that iodine from iodine-enriched Chlorella spp. is retained in muscle tissue in a higher extent than inorganic KI. Nevertheless, iodine concentrations in the meat are still low to be considered as a relevant iodine source for human nutrition.

Effects of dietary iodine and selenium on nutrient digestibility, serum thyroid hormones, and antioxidant status of Liaoning cashmere goats

Forty-eight 2-year-old Liaoning Cashmere goats (body weight = 38.0 ± 2.94 kg) were used to investigate the effects of dietary iodine (I) and selenium (Se) supplementation on nutrient digestibility, serum thyroid hormones, and antioxidant status during the cashmere telogen period to learn more about the effects of dietary I and Se on nutrition or health status of Cashmere goats. The goats were equally divided into six groups of eight animals each that were treated with 0, 2, or 4 mg of supplemental I/kg dry matter (DM) and 0 or 1 mg of supplemental Se/kg DM in a 2 × 3 factorial arrangement of treatments. The six treatments were I(0)Se(0), I(2)Se(0), I(4)Se(0), I(0)Se(1), I(2)Se(1), and I(4)Se(1). The concentrations of I and Se in the basal diet were 0.67 and 0.09 mg/kg DM, respectively. The study started in March and proceeded for 45 days. Supplemental I or Se alone had no effect on nutrient digestibility and nitrogen metabolism. However, the interaction between I and Se was significant regarding the digestibility of acid detergent fiber (ADF; P < 0.05), and compared with group I(4)Se(1), the digestibility of ADF was significantly increased in group I(4)Se(0) (P < 0.05). Selenium supplementation did not affect serum triiodothyronine (T(3)) or thyroxine (T(4)) concentrations. However, the concentration of serum T(4) but not that of T(3) was significantly increased with I supplementation (P < 0.05). In addition, serum superoxide dismutase (SOD) activity was not affected (P > 0.05), but serum glutathione peroxidase (GSH-Px) activity was significantly decreased by I supplementation (P < 0.05). The antioxidant status was improved by Se supplementation, and the activities of SOD and GSH-Px were significantly increased (P < 0.05).

Effect of iodine source and dose on growth and iodine content in tissue and plasma thyroid hormones in fattening pigs

PURPOSE

The aim of the present feeding trial with iodine was to assess pigs' growth performance and carcass characteristics, the iodine accumulation in tissues, and their influences on the thyroid hormones in plasma.

METHODS

Eighty pigs (33-115 kg body weight) were allotted to 5 dietary treatments: a control group (150 μg I/kg), two potassium iodide [KI] groups (4,000 and 10,000 μg I/kg), and two potassium iodate [KIO₃] groups (4,000 and 10,000 μg I/kg). Iodine concentration was determined in thyroid gland, liver, kidney, muscle, fat, and skin by ICP-MS. Furthermore, thyroxine (T₄) and triiodothyronine (T₃) in plasma were evaluated.

RESULTS

High dietary iodine tended to have a negative effect on younger animals' growth (average daily gain, ADG). However, during the entire growth period, the growth performance and carcass characteristics were not influenced by iodine dosages or sources. Irrespective of iodine source, higher iodine doses of diets affected higher iodine stores in all tested tissues except for abdominal fat. Thus, iodine supplementation with 10,000 μg I/kg feed significantly increased iodine content in thyroid gland (+122%), liver (+260%), kidney (+522%), muscle (+131%), and skin (+321%) compared to the control group. However, there was no significance of thyroid hormones in plasma.

CONCLUSIONS

As a result, pork and fat of pigs showed only low iodine accumulation even in the high-iodine groups. Thus, there should be no risk of an iodine excess in human nutrition and animal health, and the EU-upper level for iodine in pig feed can be maintained.

Toward an Ecosocial Epidemiological Approach to Goiter and Other Iodine Deficiency Disorders: A Case Study of India's Technocratic Program for Universal Iodization of Salt

The program of universal salt iodization (USI) was intensified in the 1990s. Unfortunately, a recent World Health Organization review finds that there was a global increase of 31.7 percent in total goiter rate from 1993 to 2003. However, the WHO review places only 1 country as severely, 13 as moderately, and 40 as mildly deficient in populations' iodine nutrition, and places 43 countries at optimal, 24 at high, and 5 at excessive levels of iodine nutrition. Thus, it is imperative to weigh the benefits and risks of intensifying USI further. The WHO review places India in the category of “adequate” iodine nutrition, but in 2005 the Government of India promulgated a universal ban on sale of non-iodized salt, calling iodine deficiency disorders (IDDs) a major public health problem. This article attempts to understand these contradictions and weigh the benefits and costs of USI. Based on a review of studies since the 1920s, the authors reconstruct the evolution of IDD control in India. Conceptual and methodological limitations challenge the evidence base and rationale of stricter implementation of USI now. Finding evidence for its negative impact, the authors recommend a reexamination of the USI strategy and propose a safer, people-centered, ecosocial epidemiological approach rather than a universal legal ban.

Reporting of systematic reviews of micronutrients and health: a critical appraisal

BACKGROUND

The quality of nutrition-related systematic reviews (SRs) is an unstudied but important factor affecting their usefulness.

OBJECTIVES

The objectives were to evaluate the reporting quality of published SRs and to identify areas of improvement.

DESIGN

Descriptive and exploratory analyses of the reporting quality (7 nutrition items and 28 SR reporting items) of all English-language SRs published through July 2007 linking micronutrients and health outcomes in humans were conducted. Factors that may be associated with reporting quality were also evaluated.

RESULTS

We identified 141 eligible SRs of 21 micronutrients. Ninety SRs that included only interventional studies met a higher proportion of our reporting criteria (median: 62%; interquartile range: 51%, 72%) than did 31 SRs with only observational studies (median: 53%; interquartile range: 47%, 60%) or 20 SRs with both study designs (median: 47%; interquartile range: 39%, 52%) (P < 0.001). SRs published after consensus reporting standards (since 2003) met a higher proportion of the reporting criteria than did earlier SRs (median: 59% compared with 50%; P = 0.01); however, the reporting of nutrition variables remained unchanged (median: 38% compared with 33%; P = 0.7). The least-reported nutrition criteria were baseline nutrient exposures (28%) and effects of measurement errors from nutrition exposures (24%). Only 58 SRs (41%) used quality scales or checklists to assess the methodologic quality of the primary studies included.

CONCLUSIONS

The reporting quality of SRs has improved 3 y after publication of SR reporting standards, but the reporting of nutrition variables has not. Improved adherence to consensus methods and reporting standards should improve the utility of nutrition SRs.