The behaviour of the thyroidal iodide trap after subtotal thyroidectomy for thyrotoxicosis and its implication for the T3-suppression test

Abstract

It is important to distinguish between symptomatic response and immunological cure in thyrotoxicosis because it has been suggested that surgery, in addition to providing a rapid symptomatic response, may also cause the disappearance of thyroid-stimulating antibodies. The evidence, however, is based largely on suppression tests which we argue may not be valid in the post-operative period. Seventy thyrotoxicosis patients were treated for 6 months with a standard course of carbimazole and T3, at the end of which each patient was classified as suppressor (S) or non-suppressor (NS) according to the fall in radioiodine uptake. Group I (18 patients) and group II (18 patients) were then randomly selected for immediate surgery while group III (34 patients) continued on antithyroid drugs. All groups were reviewed every two months from the 6th month for 12 months, during which time group I was drug-free and groups II and III received T3. Twenty-min iodide uptakes were performed in all patients at each visit to compare the serial changes in mean iodide trapping capacity between treatment groups. Despite 10-fold differences in TSH levels between groups I and II, and irrespective of suppressibility before surgery, the mean uptakes in both these groups remained basal (less than 4%) throughout the period of study, while the serial mean uptakes in group III S (no TSH, by implication no TSAB, but intact iodide trap) were consistently higher than those of group I NS (high TSH, by implication TSAB as well, but reduced iodide trap size). The data points to an absence of dose-responsiveness between TSH and the surgical-remnant's iodide trap, implying that post-thyroidectomy suppression tests (at least during the first year) cannot measure changes in iodide trapping, and therefore do not measure the same phenomenon after subtotal thyroidectomy as they do before operation when the thyroid is intact. We therefore question the validity of comparing suppressibility before and after surgery and basing the frequency of surgical cure on the result.