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Triiodothyronine Suppression Tests and TSH-Releasing Hormone Tests Before and After I-131 Therapy for Graves' Disease

Khyshu University, Fukuoka
Kuma Hospital, Kobe
University of Tokyo, Hongo, Tokyo, Japan

Correspondence: For reprints contact: S. Nagataki, MD, Third Dept. of Internal Medicine, Univ. of Tokyo, Hongo, Tokyo 113, Japan.

ABSTRACT

T₃-suppression and TRH tests were repeatedly performed in 63 patients with Graves' disease before, and 6, 12, 18, and 24 mo after I-131 therapy. These patients were selected from more than 200 I-131-treated patients; they satisfied the criteria of clinical euthyroidism, with normal serum T₄ concentrations at least during the period between 6 mo and 2 yr after the therapy. The numbers of T₃-suppressible patients increased, but only 27 of the 63 patIents (43%) were suppressible at 2 yr after therapy. Those responding to TRH also increased, and 36 of 63 patients (57%) responded to TRH at 2 yr after therapy. Most of T₃-suppressible patients were TRH responsive. Although serum T₄ concentrations were within the normal range, serum T₃ levels were above normal in almost one third of these patients, and most of those with high serum T₃ levels were T₃-nonsuppressible and TRH-nonresponding. Investigation of changes in T₃-suppressibility and TRH-responsiveness in individual patients revealed that although incidence of T₃-suppression and TRH responsiveness increased, seven patients became T₃-nonsuppressible and ten patients TRH-nonresponding within 12 mo of the time when they had been T₃-suppressible or TRH-responsive. Among TRH-responders, the number with exaggerated response to TRH increased gradually and reached 28 of 36 patients (78%) at 2 hr after therapy.

These results suggest that in Graves' patients with normal serum T₄ concentrations after I-131 therapy: (a) incidence of T₃-suppressibility and TRH-responsiveness increases and reaches 50% even 2 yr after the therapy, and that serum T₃ levels are high in T₃-nonsuppressible and TRH-nonresponding patients; (b) the results of both tests at 6 mo after therapy are not prognostically reliable; (c) latent hypothyroidism begins within 2 yr after I-131 therapy even in patients with normal serum T₄ and T₃ concentrations; and (d) failure to respond to TRH, or to T₃-suppression, is not proof that a patient requires further treatment with I-131.