Abstract
SUMMARY: Unlike medullary thyroid carcinomas, follicular cell-derived thyroid malignancies have rarely been associated with paraneoplastic endocrine syndromes. An ultrarare case of a middle-aged man with heavily treated broadly metastatic radioactive iodine-refractory widely invasive Hürthle cell carcinoma (HCC) of the thyroid with two synchronous paraneoplastic endocrine syndromes, T3 thyrotoxicosis and hypercalcemia of malignancy, is discussed here. The levothyroxine-induced T3 thyrotoxicosis was a gradual process that became more noticeable as the tumor burden, refractory to different modalities of therapy, expanded. The 1,25-dihydroxyvitamin-D-mediated hypercalcemia, on the other hand, developed in a manner of weeks, as it usually happens. It is important to emphasize that in patients with metastatic Hürthle cell and follicular carcinomas of the thyroid, on TSH suppressive therapy, the unexplained and progressive decline in FT4 and rise in FT3 levels, resulting in an elevated FT4/FT3 ratio, could be an indication of augmented type 1 (D1) and/or type 2 (D2) deiodinase expression in tumoral tissue, causing an increased conversion from the prohormone T4 into the active metabolite T3 via outer ring deiodination. LEARNING POINTS: Albeit extremely rare, some patients with thyroid cancer can present with more than one concomitant paraneoplastic syndrome. Although medullary thyroid carcinoma is the thyroid malignancy that is usually associated with paraneoplastic endocrine syndromes, follicular cell-derived thyroid cancers have been rarely described as being the culprit. In patients with metastatic Hürthle cell and follicular thyroid carcinomas, the unexplained and progressive decline in FT4 and rise in FT3 levels could be an indication of augmented type 1 (D1) and/or type 2 (D2) deiodinase expression in tumoral tissue, causing an increased conversion from T4 into T3 leading to T3 thyrotoxicosis.