Thyroid suppression test with a single oral dose of levothyroxine in the diagnosis of functional thyroid autonomy

To propose a modified form of thyroid suppression test with use of a single oral dose of levothyroxine (35 mg/kg).

After a baseline scintigram, 23 patients with nodular goiter suspected of autonomous function (warm or hot nodules, subnormal or undetectable thyrotropin levels, or both findings) and 14 normal subjects underwent a repeated scintigram 4 days after administration of levothyroxine. We evaluated triiodothyronine (T(3)), free thyroxine, and thyrotropin before and on the first, second, third, fourth, and seventh days after administration of the individualized dose of levothyroxine.

The 99th percentile of postsuppression uptake in normal subjects was determined, and an uptake >12.4%, a 131 I concentration restricted to the nodule, or both factors were adopted as the criteria for diagnosis of an autonomously functioning thyroid nodule. Twelve patients were considered to have autonomously functioning nodules, and 11 patients were considered to have nonautonomous nodules. Baseline thyrotropin levels in patients with autonomous nodules did not differ significantly from those in patients with nonautonomous nodules. No signs or symptoms of toxicity were detected during the test, but all study subjects had increased free thyroxine values, and seven had high levels of T(3).

The thyroid suppression test with 35 mg/kg of levothyroxine is an effective method for the diagnosis of an autonomously functioning thyroid nodule, is nontoxic, and avoids the inaccurate use of the medication occasionally observed with T(3). Even sensitive methods of thyrotropin determination cannot replace this test in the evaluation of autonomous thyroid function.