3,5,3′-Triiodothyronine Thyrotoxicosis due to Increased Conversion of Administered Levothyroxine in Patients with Massive Metastatic Follicular Thyroid Carcinoma

Abstract

Twenty percent of patients with massive metastatic FC exhibited T(3) thyrotoxicosis, most likely due to increased conversion of T(4) to T(3) by tumor expressing high D1 and D2 activities. Occasional measurement of serum FT(3) in addition to FT(4) and TSH is recommended in patients with massive metastatic FC, especially when serum FT(4) decreases on fixed doses of levothyroxine.