Global or Gaelic epidemic of hypothyroidism?

Treatment for primary hypothyroidism: current approaches and future possibilities

Primary hypothyroidism is the most common endocrine disease. Although the diagnosis and treatment of hypothyroidism is often considered simple, there are large numbers of people with this condition who are suboptimally treated. Even in those people with hypothyroidism who are biochemically euthyroid on levothyroxine replacement there is a significant proportion who report poorer quality of life. This review explores the historical and current treatment options for hypothyroidism, reasons for and potential solutions to suboptimal treatment, and future possibilities in the treatment of hypothyroidism.

Effect of iodine supplementation during pregnancy on infant neurodevelopment at 1 year of age

Iodine is the main constituent of thyroid hormones, which in turn are required for fetal brain development. However, the relation between iodine intake during pregnancy, thyroid function, and child neurodevelopment needs further evaluation. The authors assessed the association of maternal iodine intake from diet and supplements during pregnancy and of maternal and neonatal thyroid function with infant neurodevelopment. The Mental Development Index and Psychomotor Development Index (PDI) for 691 children were obtained between 2005 and 2007 using the Bayley Scales of Infant Development at age 1 year in a prebirth cohort in Valencia, Spain. In multivariate analyses, a maternal thyrotropin level >4 μU/mL was associated with an increased risk of a PDI <85 (odds ratio = 3.5, P = 0.02). Maternal intake of ≥150 μg/day, compared with <100 μg/day, of iodine from supplements was associated with a 5.2-point decrease in PDI (95% confidence interval: -8.1, -2.2) and a 1.8-fold increase in the odds of a PDI <85 (95% confidence interval: 1.0, 3.3). When analyses were stratified by sex, this association was intensified for girls but was not observed for boys. Further evidence on the safety and effectiveness of iodine supplementation during pregnancy is needed before it is systematically recommended in iodine-sufficient or mildly deficient areas.

Iodine intake as a determinant of thyroid disorders in populations

Depending on the availability of iodine, the thyroid gland is able to enhance or limit the use of iodine for thyroid hormone production. When compensation fails, as in severely iodine-deficient populations, hypothyroidism and developmental brain damage will be the dominating disorders. This is, out of all comparison, the most serious association between disease and the level of iodine intake in a population. In less severe iodine deficiency, the normal thyroid gland is able to adapt and keep thyroid hormone production within the normal range. However, the prolonged thyroid hyperactivity associated with such adaptation leads to thyroid growth, and during follicular cell proliferation there is a tendency to mutations leading to multifocal autonomous growth and function. In populations with mild and moderate iodine deficiency, such multifocal autonomous thyroid function is a common cause of hyperthyroidism in elderly people, and the prevalence of thyroid enlargement and nodularity is high. The average serum TSH tends to decrease with age in such populations caused by the high frequency of autonomous thyroid hormone production. On the other hand, epidemiological studies have shown that hypothyroidism is more prevalent in populations with a high iodine intake. Probably, this is also a complication to thyroid adaptation to iodine intake. Many thyroid processes are inhibited when iodine intake becomes high, and the frequency of apoptosis of follicular cells becomes higher. Abnormal inhibition of thyroid function by high levels of iodine is especially common in people affected by thyroid autoimmunity (Hashimoto's thyroiditis). In populations with high iodine intake, the average serum thyroid-stimulating hormone (TSH) tends to increase with age. This phenomenon is especially pronounced in Caucasian populations with a genetically determined high tendency to thyroid autoimmunity. A small tendency to higher serum TSH may be observed already when iodine intake is brought from mildly deficient to adequate, but there is at present no evidence that slightly elevated serum TSH in elderly people leads to an increase in morbidity and mortality.

CONCLUSION

Even minor differences in iodine intake between populations are associated with differences in the occurrence of thyroid disorders. Both iodine intake levels below and above the recommended interval are associated with an increase in the risk of disease in the population. Optimally, iodine intake of a population should be kept within a relatively narrow interval where iodine deficiency disorders are prevented, but not higher. Monitoring and adjusting of iodine intake in a population is an important part of preventive medicine.

Can neonatal TSH screening reflect trends in population iodine intake?

BACKGROUND

The distribution of neonatal blood thyroid-stimulating hormone (TSH) concentrations has been used as an index reflecting population dietary iodine intake, with higher concentrations being indicative of lower iodine intake. We examined this distribution in neonates born in Ireland, where the pregnant population has shown a recent decline in urinary iodine (UI) excretion. Our objectives were to determine if any alteration was observed in the percentage of values > 5.0 mIU/L and whether a trend in neonatal blood TSH was apparent.

METHODS

Samples drawn from the National Neonatal Screening Programme were assessed during the years 1995-2006 from winter (January n = 35,079) and summer (August n = 37,940) months, respectively, in view of the known seasonal variation in Irish dietary iodine intake.

RESULTS

Apart from the first years studied (1995-1996), the proportion of individual blood TSH values >5.0 mIU/L did not exceed 3%, a value believed to be indicative of iodine deficiency. A significant declining trend in the proportion of blood TSH >5.0 mIU/L was observed in subsequent years (p < 0.01). While excluding severe iodine deficiency, these analyses failed to detect the slight but highly significant (p < 0.001) tendency toward increasing blood TSH within the 0-5.0 mIU/L interval in the study population between 1999 and 2006, which was greater in summer than in winter months (p < 0.001).

CONCLUSIONS

These data support a link between fetal thyroid function and a fall in maternal iodine intake. While the findings of the proportion of blood TSH values >5.0 mIU/L exclude severe maternal or fetal iodine deficiency, a trend toward increasing TSH may provide an early indication of impending iodine deficiency. The findings assume greater importance in the context of declining UI reported from many developed countries even where the proportion of blood TSH values >5.0 mIU/L is <3%, thus excluding severe maternal and fetal iodine deficiency.

Increasing prevalence and incidence of thyroid disease in Tayside, Scotland: the Thyroid Epidemiology Audit and Research Study (TEARS)

OBJECTIVE

We aimed to describe the changing incidence of thyroid disease in a population-based study in Tayside, Scotland (population 390 000) between 1994 and 2001.

DESIGN

A retrospective, data-linkage, population-based study measuring the incidence and prevalence of thyroid disease.

PATIENTS

All patients with newly diagnosed, treated and stable thyroid disease in Tayside were identified by electronic linkage of six datasets, including all regional biochemistry data, hospital admissions, deaths and a thyroid follow-up register.

RESULTS

The overall prevalence of thyroid dysfunction has increased from 2.3% to 3.8% (1994-2001). The prevalence of ever having had hyperthyroidism increased from 0.86% to 1.26% in females and 0.17% to 0.24% in males (P < 0.0001 for both). The standardized incidence of hyperthyroidism increased from 0.68 to 0.87 per 1000 females/year, representing a 6.3% annual increase (P < 0.0001). The prevalence of primary hypothyroidism increased from 3.12% to 5.14% in females and 0.51% to 0.88% in males (P < 0.0001 for both). The standardized incidence of primary hypothyroidism did not change and varied between 3.90 and 4.89 per 1000 females/year over the 8 years. Incidence of hypothyroidism in males increased from 0.65 to 1.01 per 1000 males/year (P = 0.0017). Mean age at diagnosis of primary hypothyroidism declined in females from 1994 to 2001.

CONCLUSIONS

The prevalence of primary hypothyroidism and previous hyperthyroidism has increased in Tayside, Scotland. This is partly due to an increasing incidence of disease, increased ascertainment and earlier diagnosis of disease. This will result in an increased workload for endocrinologists and general practitioners.

Mortality and vascular outcomes in patients treated for thyroid dysfunction

CONTEXT

There are limited studies describing mortality and morbidity in patients treated for hyperthyroidism, and no data exist for people with treated hypothyroidism.

OBJECTIVE

The objective of the study was to describe all-cause mortality and vascular mortality and morbidity in patients after treatment for hyperthyroidism and hypothyroidism.

DESIGN

This was a population-based cohort study from 1994 to 2001.

SETTING

The study was conducted in Tayside, Scotland.

PATIENTS

All patients were treated for thyroid dysfunction.

INTERVENTION(S)

Event rates among patients with thyroid dysfunction were compared with rates in the general population. We measured standardized mortality ratio and standardized incidence ratio (SIR).

MAIN OUTCOME MEASURE(S)

The primary outcome was all-cause mortality. The secondary outcome was serious vascular event, the composite end point of nonfatal myocardial infarction, nonfatal stroke, or vascular death.

RESULTS

There were 15,889 primary hypothyroid and 3,888 hyperthyroid patients. There were 3,116,719 patient-years of follow-up in 524,152 subjects in the general population. No increase was found in all-cause mortality or serious vascular events in patients with treated hypothyroidism or hyperthyroidism. Nonfatal ischemic heart disease [SIR 1.23, 95% confidence interval (CI) 1.10-1.36] and dysrhythmias (SIR 1.32, 95% CI 1.11-1.57) were increased in treated hypothyroidism when adjusted for age, sex, diabetic status, and previous vascular disease. In treated stabilized hyperthyroidism, only the risk of dysrhythmias was increased (SIR 2.71, 95% CI 1.63-4.24). Risk of heart failure or cerebrovascular disease was not increased in either patient group.

CONCLUSIONS

We found no increase in all-cause mortality in subjects with treated thyroid disease. However, there was increased risk of cardiovascular morbidity in patients with treated primary hypothyroidism and dysrhythmias in treated hyperthyroidism.