Triiodothyronine Withdrawal Test—A Test of Thyroid-Pituitary Adequacy

ABSTRACT A test for the adequacy of pituitary-thyroid function is proposed utilizing the magnitude of the rebound observed in 24-hr 131I thyroid uptake (RAIU) and the protein-bound iodine (PBI) values 10 days following withdrawal from full suppressive doses of orally administered triiodothyronine (T3). A rebound RAIU value exceeding 16% (the mean base line value is 16% in this laboratory) and/or a rise in the PBI of greater than 1.0 μg/100 ml was consistent with pituitary-thyroid function which was adequate enough to ultimately maintain a normal serum PBI level. The subjects who were studied with either primary or secondary hypothyroidism uniformly failed to achieve these rebound criteria. This T3 withdrawal test is particularly well designed for the evaluation of thyroid function in the nongoitrous patient on long-term thyroid hormone replacement therapy for an unsubstantiated diagnosis of hypothyroidism.