Differentiated Thyroid Cancer in Two Patients with Resistance to Thyroid Hormone

Background: Resistance to thyroid hormone (RTH) is a genetic disease characterized by a\r\nreduced responsiveness of the pituitary and peripheral target tissues to thyroid hormone. We\r\ndescribe two patients with RTH in whom differentiated thyroid cancer (DTC) was diagnosed.\r\nPatient findings: In both patients RTH was unequivocally diagnosed and both underwent\r\nthyroidectomy for multinodular goiter. In Patient # 1, histology showed a papillary thyroid\r\ncarcinoma pT2. Because of serum TSH levels were elevated even while the patient was taking 150\r\nμg daily of levothyroxine (LT4), the patient was treated with 131I 100 mCi for ablation of the\r\nthyroid remnant without discontinuing his LT4 therapy. We obtained a clinically adequate response\r\nby administering LT4 175 μg/day (2.18 μg/kg), but the serum TSH was persistently elevated on this\r\ndose. The patient was considered free of disease after eight years of follow-up. In Patient # 2,\r\nhistology revealed a papillary microcarcinoma (0.6 cm). Diagnostic whole-body-scan was\r\nperformed while the patient was taking 100 μg/day LT4, a time that his serum TSH was 38 μU/ml).\r\nOnly a small remnant was revealed so 131I remnant ablation was not performed. While taking LT4\r\nat a dose of 175 μg/day (3 μg/kg), the serum TSH was persistently high, serum thyroid hormone\r\nlevels were in the normal-high range and he appeared to be clinically euthyroid. There has been no\r\nevidence of persistent or recurrent thyroid carcinoma in ultrasonography and Tg measurements that\r\nhave been performed on a yearly basis for three years.\r\nConclusion: Patients with thyroid carcinoma and RTH are a unique model of thyroid cancer\r\nwhere follow-up likely occurs in the setting of constantly elevated serum TSH concentrations. The\r\nconcern in these patients is that their persistent elevation of serum TSH may have an adverse effect\r\non their thyroid cancer and management choices in terms of the dose of LT4 that provides the\r\noptimum lowering of serum TSH without toxicity are difficult, particularly in the situation where,\r\nas was the case with one of our patients, there was cardiac disease.