Nicotinic Acid Decreases Serum Thyroid Hormone Levels While Maintaining a Euthyroid State

To evaluate the effects of nicotinic acid on serum thyroid hormone levels in the absence of systemic illness or hepatic dysfunction. 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. In the five study patients, we measured serum lipids in conjunction with serum thyroxine (T₄), triiodothyronine (T₃) resin uptake, T₃, free T₄, thyroid-stimulating hormone (TSH), and thyroxine-binding globulin before, during, and after treatment with nicotinic acid. Serum lipid levels responded appropriately to nicotinic acid treatment. Thyroid function studies done a mean of 1.3 years (range, 0.5 to 3.5) after initiation of nicotinic acid therapy (mean daily dose, 2.6 ± 0.7 g) revealed significant decreases in serum levels of total T₄ (21 %), free T₄ index (16%), T₃ (13 %), and thyroxine-binding globulin (23 %) (P<0.02) whereas no significant changes were noted in free T₄, T₃ resin uptake, and TSH levels. During the course of treatment, the patients, who were carefully questioned, had no symptoms of hypothyroidism. Hypothyroidism was further excluded in three patients who had a normal serum TSH response to administration of thyrotropin-releasing hormone. In two patients, measurements of thyroid function returned to pretreatment levels after discontinuation of nicotinic acid therapy. No patient had significant abnormalities in liver-associated enzymes or evidence of systemic illness during the course of treatment. These results suggest that nicotinic acid decreases serum thyroid hormone concentrations while maintaining a euthyroid state. This effect may be mediated through reduction in thyroxine-binding globulin, but other mechanisms may also be involved.