High T3 Syndrome Associated with Metastatic Papillary and Poorly Differentiated Thyroid Cancer

Circulating levels of free thyroxine (T4) and triiodothyronine (T3) are tightly regulated by thyrotropin (TSH) via deiodinases. Free T4 and T3 levels are usually congruent but may become discordant when deiodinase activities are altered by diseases or medications. High T3 syndrome is seen in hyperthyroidism, Allan-Herndon-Dudley syndrome, and massive metastatic follicular thyroid cancer (FTC), where the T4-to-T3 conversion is increased (1-3). High T3 syndrome has not been reported in metastatic papillary thyroid cancer (PTC) or poorly differentiated thyroid cancer (PDTC). Described here is the first case of high T3 syndrome in a patient with a large metastatic burden of PTC and PDTC.