The effect on thyroid status of changing from thyroid USP to sodium L-thyroxine was evaluated in 40 patients. With thyroid, abnormally high triiodothyronine (T3) levels were seen in 36 of 38 patients receiving doses of 90 to 240 mg; compared to sodium L-thyroxine, 0.15 to 0.2 mg, the serum T3 was higher (289 +/- 15 ng/dl versus 176 +/- 9 ng/dl, p less than 0.0005) and the thyroxine (T4) lower (7.4 +/- 0.3 microgram/dl versus 11.6 +/- 0.5 microgram/dl, P less than 0.01). Thyrotoxic symptoms occurred in six patients and diminished or disappeared after the change to sodium L-thyroxine, suggesting that the raised T3 level with thyroid may have undesirable effects in some patients. The T4 level, because it is low whether symptoms are present or not, may inadvertently suggest the need for higher dosage of desiccated thyroid in patients who have already received adequate replacement. The dose of sodium L-thyroxine was adequately assessed by measurement of both T4 and T3 levels. Thyroid USP should be discontinued as thyroid medication since it produces thyroid hormone levels that are misleading estimates of thyroid function and can cause thyrotoxic symptoms.