Isolated high serum triiodothyronine levels. Clinical relevance after sodium iodide I 131 therapy.

Ten of 25 thyrotoxic patients treated with sodium iodine I 131 had thyroid function test results that indicated a high triiodothyronine (T3) level, with normal thyroxine (T4) and thyroid-stimulating hormone (TSH) levels, within 15 months of treatment. This pattern was usually transitional and lasted a variable time, eventually leading to euthyroidism, hyperthyroidism, or hypothyroidism. It is concluded that an isolated elevation of T3 levels after radioactive iodine therapy can be associated with any clinical pattern of thyroid function and has no clear prognostic value. No specific treatment is needed for this abnormality.