Triumphs of the thyroid despite lesser conversion

Thyroid hormone replacement therapy: three 'simple' questions, complex answers

Current guidelines recommend that hypothyroid patients should be treated with levothyroxine, which in the vast majority of the cases leads to resolution of the symptoms and normalization of serum free T4 (FT4), T3 and TSH levels. However, a small group of hypothyroid patients remain symptomatic for neurocognitive dysfunction despite normal serum FT4 and TSH, which could be explained by localized brain hypothyroidism. More than half of the T3 in the brain is produced locally via the action of the type II deiodinase (D2) and variability/defects in this pathway could explain the residual symptoms. If this rationale is correct, adding liothyronine to the replacement therapy could prove beneficial. However, with a few exceptions, several clinical trials failed to identify any beneficial effects of combined therapy. More recently, the results of a large clinical trial revealed a better neurocognitive outcome with combined therapy only in hypothyroid patients carrying a polymorphism in the DIO2 gene. This obviously needs to be confirmed by other groups but it is tempting to speculate that combined levothyroxine and liothyronine has a place in the treatment of hypothyroidism, for some.

Selenium, selenoproteins and the thyroid gland: interactions in health and disease

The trace element selenium is an essential micronutrient that is required for the biosynthesis of selenocysteine-containing selenoproteins. Most of the known selenoproteins are expressed in the thyroid gland, including some with still unknown functions. Among the well-characterized selenoproteins are the iodothyronine deiodinases, glutathione peroxidases and thioredoxin reductases, enzymes involved in thyroid hormone metabolism, regulation of redox state and protection from oxidative damage. Selenium content in selenium-sensitive tissues such as the liver, kidney or muscle and expression of nonessential selenoproteins, such as the glutathione peroxidases GPx1 and GPx3, is controlled by nutritional supply. The thyroid gland is, however, largely independent from dietary selenium intake and thyroid selenoproteins are preferentially expressed. As a consequence, no explicit effects on thyroid hormone profiles are observed in healthy individuals undergoing selenium supplementation. However, low selenium status correlates with risk of goiter and multiple nodules in European women. Some clinical studies have demonstrated that selenium-deficient patients with autoimmune thyroid disease benefit from selenium supplementation, although the data are conflicting and many parameters must still be defined. The baseline selenium status of an individual could constitute the most important parameter modifying the outcome of selenium supplementation, which might primarily disrupt self-amplifying cycles of the endocrine-immune system interface rectifying the interaction of lymphocytes with thyroid autoantigens. Selenium deficiency is likely to constitute a risk factor for a feedforward derangement of the immune system-thyroid interaction, while selenium supplementation appears to dampen the self-amplifying nature of this derailed interaction.

Hypothyroidism and subclinical hypothyroidism in the older patient

OBJECTIVE

To review the etiology, precipitating factors, clinical findings, screening recommendations, and treatment for primary hypothyroidism and subclinical hypothyroidism in the older patient.

DATA SOURCES

A PubMed search of English language articles using a combination of words: elderly, thyroid, hypothyroid,* subclinical hypothyroid,* etiology, screening, diagnosis, and treatment to identify original studies, guidelines, and reviews on primary hypothyroidism and subclinical hypothyroidism published between 1979 and present.

STUDY SELECTION AND DATA EXTRACTION

Overall, 51 clinical reviews, original studies, references, and guidelines were obtained and evaluated on their clinical relevance to the older patient population.

DATA SYNTHESIS

The literature included guidelines and considerations for the diagnosis, screening, and management of subclinical and overt primary hypothyroidism in the older patient.

CONCLUSION

Females and individuals 60 years of age or older have a higher prevalence of primary hypothyroidism and subclinical hypothyroidism. While screening recommendations exist, the need or suggested age to initiate screening varies among organizations. TSH and free T4 values are used for diagnosing and monitoring. Levothyroxine remains the drug of choice for replacing endogenous thyroid hormone. Despite evidence to suggest its need, the treatment of subclinical hypothyroidism remains controversial.