Central hypothyroidism is an exceedingly rare form of hypothyroidism that results from a variety of conditions affecting the hypothalamus and the pituitary gland. The classic biochemical abnormality seen in these patients includes a low serum level of circulating thyroxine (T4) concomitant with an inappropriately low level of thyrotropin. Because patients with isolated triiodothyronine (T3) toxicosis also present with this biochemical pattern, it is important to measure T3 levels in such patients before making a diagnosis of central hypothyroidism.