ANOMALOUS PLASMA TSH LEVELS IN PATIENTS DEVELOPING HYPOTHYROIDISM IN THE EARLY MONTHS AFTER131l THERAPY FOR THYROTOXICOSIS

Abstract

Serial measurements of serum T3, T4 and plasma TSH were made in seven patients developing hypothyroidism in the early months after ¹³¹I therapy for thyrotoxicosis. In each patient a low serum T4 was found in the presence of a normal plasma TSH before the onset of clinical hypothyroidism. In three of the patients the normal plasma TSH was associated with normal or high serum T3, but in the other four patients both serum T3 and T4 were low in association with a normal plasma TSH. Such a situation may reflect the slow response of the brain-thyroid axis recently exposed to high circulating levels of thyroid hormone, analogous to the suppression of the brain-adrenal axis after prolonged exposure to high circulating levels of corticosteroids.