Peripheral Metabolism of Thyroid Hormones: A Review

Alternative Medicine Review, August, 2000 by Greg Kelly

This study suggests zinc status influences peripheral metabolism of thyroid hormones. Since zinc is not a cofactor in hepatic type I-deiodinase enzyme, the nature of zinc's influence on aspects of peripheral metabolism in animals and humans remains unclear.

Nutrients with Potential Influence on Thyroid Hormone Metabolism: Vitamins

Niacin

Evidence suggests niacin supplementation can influence thyroid hormone levels in at least some individuals. One author reported cytopenia and hypothyroxinemia with a concomitant decrease in thyroxine-binding globulin in two patients receiving niacin. All thyroid function tests returned to normal after niacin supplementation was discontinued.[118]

The impact of sustained supplementation with niacin (mean daily dose of 2.6 grams for an average duration of 1.3 years) was observed in one female and four male subjects with hyperlipidemia. Before and after thyroid function studies revealed significant decreases in serum levels of T4, T3, and TBG, with no significant alterations in fT4 and TSH levels. Discontinuation of niacin resulted in a return to pretreatment levels of these parameters of thyroid function.[119]

While results suggest niacin can influence serum thyroid hormone concentrations, it is currently not known whether this is a centrally-mediated result, a direct result of a decrease in TBG, or a niacin-induced alteration in some aspect of peripheral conversion. However, since TSH was unaltered, evidence suggests an influence outside the hypothalamic-pituitary-thyroid axis.

Vitamin B12

In animals, a tissue vitamin B 12 deficiency was associated with a slight reduction of type I 5'-deiodinase activity in liver and with a significant reduction of the T3 level in serum.[120] Studies in human subjects have not been conducted; however, it is possible that a vitamin B 12 deficiency might provoke a similar detrimental influence on peripheral activation of T3 from T4.

Lipoic Acid

Lipoic acid appears to influence the metabolic fate of T4 when co-administered with T4 therapy. Administration of T4 for 22 days resulted in a substantial increase in serum T3 concentrations; however, when lipoic acid was given in conjunction with T4 therapy for nine days a 56-percent suppression of the expected T4-induced increase in generation of T3 was observed (although T3 levels were elevated above control levels). Continuous supplementation of lipoic acid during T4 treatment resulted in a continued lower production of T3 than would have been expected from T4 therapy.[121]

While the authors suggest lipoic acid might exert an influence on peripheral tissue deiodinase activity, it is also possible this nutrient might have influenced conjugation reactions. It is currently not known whether lipoic acid supplementation influences thyroid hormone metabolism in normal individuals who are not receiving T4 therapy. Since it is usually not a therapeutic advantage to decrease peripheral activation of T3 subsequent to T4 therapy, use of this supplement in hypothyroid patients receiving exogenous hormone therapy should be approached with caution.