Metastatic Follicular Thyroid Carcinoma as a Cause of Low Serum Thyroxine with a Normal Thyroid Stimulating Hormone Level

Thyroid function is usually normal in differentiated thyroid carcinoma. We describe a case of a female patient who had metastatic follicular thyroid carcinoma (FTC) to the spine and lungs, who was clinically euthyroid but had very low free tetraiodothyronine (fT4) and normal thyroid stimulating hormone (TSH). Free triiodothyronine (fT3) and total T3 (TT3) were normal. Levothyroxine treatment increased fT4 marginally but caused a two- to three-fold rise in fT3 and TT3 along with suppressed TSH. This is likely due to hyperconversion of T4 to T3 from elevation in D2 deiodinase activity in the tumor. This phenomenon has been reported to occur in about 20% of metastatic FTC.