Thyroxine to Triiodothyronine Hyperconversion Thyrotoxicosis in Patients with Large Metastases of Follicular Thyroid Carcinoma

Objective: We experienced two cases of follicular thyroid carcinoma with distant metastases, which showed high levels of free triiodothyronine (T₃) while free thyroxine (T₄) levels remained in the low or normal range. In this report, we described the detail of these cases and examined the cause of T₃ thyrotoxicosis. Design: In one of the cases, quantitative measurement of types I and II iodothyronine deiodinase mRNAs was performed using a surgically dissected tissue from the primary tumor and a distant metastasis. Main outcome: In the both tissues, types I and II iodothyronine deiodinase mRNAs were expressed in the same level as in the normal thyroid tissues.Conclusion: These results suggest that T₃ thyrotoxicosis in our patients was caused by hyperconversion of administered levothyroxine to T₃. In the follow-up of patients with distant metastases of follicular carcinoma, not only free T₄, but also free T₃ should be tested to avoid the excessive administration of levothyroxine.