Thyroid function in patients with Graves' disease long after 131I therapy

Serum level of thyroid hormones and thyrotropin (TSH) was measured by radioimmunoassay in 63 cases of hyperthyroid patients more than 10 years after 131I therapy. Serum level of both 3, 5, 3'-triiodothyronine (T3) and thyroxine (T4) was normal in 37 patients (58.7%). In this group, about 40% of the patients showed mildly elevated serum level of TSH. However, serum level of T3, T4 and 3, 3', 5'-triiodothyronine (rT3) in these TSH-elevated patients was almost identical with that of TSH-normal euthyroid patients. Therefore, mild elevation of the serum level of TSH seemed to well maintain the peripheral thyroid hormone levels at first. As the thyroidal impairment became severer, the serum level of TSH elevated remarkably, and that of T3, T4 and rT3 began to decrease conversely, indicating thyroidal failure to maintain peripheral thyroid hormone levels in spite of the extreme TSH stimulation. At that stage, preferential preservation of serum level of T3 was suggested by the increase in serum T3/T4 ratio and the decrease in serum rT3/T3 ratio. On the other hand, serum rT3/T4 ratio remained fairly constant even in 8 patients (12.7%) with low serum level of both T3 and T4. The preferential synthesis of T3 in the 131I-irradiated thyroid gland was suspected. However, experimental study failed to show the evidence for that. At least 2 of the eight patients with normal T3 but subnormal T4 level seemed to be clinically hypothyroid and responded dramatically to the replacement therapy, suggesting that they were already hypothyroid in spite of the normal serum level of T3. (author)