This report compares three patients with severe nonthyroidal illness and serum thyrotropin concentrations in the range normally associated with hypothyroidism (greater than 20 microU/ml) and a severely ill patient with primary hypothyroidism. No evidence for primary thyroid disease was found among the severely ill patients. Serial studies showed that, in general, the elevated thyrotropin level was associated with an increase in serum thyroxine concentration. Serum free thyroxine concentration, triiodothyronine-to-thyroxine ratio, and thyrotropin response to thyrotropin-releasing hormone were in the range reported in severe nonthyroidal illness and clearly different from that in the severely ill patient with primary hypothyroidism. Serum thyrotropin concentration in severe nonthyroidal illness may be elevated in the range previously thought to be diagnostic of primary hypothyroidism.