NICOTINIC-ACID DECREASES SERUM THYROID-HORMONE LEVELS WHILE MAINTAINING A EUTHYROID STATE

Objective: To evaluate the effects of nicotinic acid on serum thyroid hormone levels in the absence of systemic illness or hepatic dysfuncti on. Design: We determined the effect of treatment with nicotinic acid on serum thyroid hormone levels in one female and four male patients ( mean age, 44.4 years) with hyperlipidemia. Material and Methods: In th e five study patients, we measured serum lipids in conjunction with se rum thyroxine (T-4), triiodothyronine (T-3) resin uptake, T-3, free T- 4, thyroid-stimulating hormone (TSH), and thyroxine-binding globulin b efore, during, and after treatment with nicotinic acid. Results: Serum lipid levels responded appropriately to nicotinic acid treatment. Thy roid function studies done a mean of 1.3 years (range, 0.5 to 3.5) aft er initiation of nicotinic acid therapy (mean daily dose, 2.6 +/- 0.7 g) revealed significant decreases in serum levels of total T-4 (21%), free T-4 index (16%), T-3 (13%), and thyroxine-binding globulin (23%) (P < 0.02), whereas no significant changes mere noted in free T-4, T-3 resin uptake, and TSH levels. During the course of treatment, the pat ients, who were carefully questioned, had no symptoms of hypothyroidis m. Hypothyroidism was further excluded in three patients who had a nor mal serum TSH response to administration of thyrotropin releasing horm one. In two patients, measurements of thyroid function returned to pre treatment levels after discontinuation of nicotinic acid therapy. No p atient had significant abnormalities in liver-associated enzymes or ev idence of systemic illness during the course of treatment. Conclusion: These results suggest that nicotinic acid decreases serum thyroid hor mone concentrations while maintaining a euthyroid state. This effect m ay be mediated through reduction in thyroxine-binding globulin, but ot her mechanisms may also be involved.