Lack of TSH inhibition by exogenous L-T4 in a patient with follicular thyroid carcinoma.

We report a case of a male patient thyroidectomized for follicular thyroid carcinoma and presenting extremely elevated serum thyrotropin levels under L-T4 suppressive therapy. Administration of L-T3 in increasing amounts resulted in a significant decrease of serum TSH levels. The nature of the possible molecular defects underlying this unusual condition and pitfalls arising from the failure of L-T4 therapy to inhibit TSH secretion in a patient in post-surgical follow-up for follicular carcinoma are discussed.