Clofibrate and a related compound suppress TSH secretion in primary hypothyroidism

A single oral dose of the hypolipidaemic agent ethyl-p-chlorophenoxyisobutyrate (clofibrate, 750 mg) produced a significant reduction on the high basal serum TSH level in patients with primary hypothyroidism. There was no consistent change in serum levels of thyroxine-iodine (T4-I), triiodothyronine (T3) and per cent free T4 (%FT4) during the study. On the other hand, clofibrate failed to produce discernible changes in the basal and TRH-induced TSH secretion in euthyroid subjects. Similar results were also obtained with the centrally active drug meclofenoxate hydrochloride (MH, 750 mg, drip infusion), similar in structure to clofibrate. These findings suggest that clofibrate and MH inhibit TSH secretion in patients with primary hypothyroidism, possibly by a direct action at the hypothalamic or pituitary level.