We present a case of a 63 year old man who developed T3-toxicosis 13 years following thyroidectomy and ablative radioiodine therapy for follicular thyroid cancer (FTC). Although asymptomatic (free T4 18.4 pmol/l, TSH <0.01 mU/l) while taking levothyroxine (T4) 200 μg/d during long-term suppressive thyroxine therapy he later presented with uncontrolled atrial fibrillation and congestive cardiac failure. Thyrotoxicosis due to excessive levothyroxine intake was considered however free T4 levels were similar to previous measurements (14.7 pmol/l). The possibility of isolated T3-toxicosis was suspected and confirmed (free T3 levels varying between 10.5-23.8 nmol/l). Iodine-avid pulmonary and hepatic metastases were visualized on I123 scanning, in the absence of neck uptake, and thyroxine therapy was withdrawn prior to further treatment of 3700 MBq I131.
Comment
Thyrotoxicosis caused by thyroid cancer occurs rarely and is usually due to follicular neoplasia, often in association with pulmonary and/or bony metastatic disease. As in this case, the biochemical profile is frequently that of isolated T3-toxicosis. While treatment and survival of cases with thyrotoxicosis appear similar to euthyroid cases with FTC, development of hyperthyroidism implies a large tumour bulk due to reduced iodine concentrating efficiency. We await clinical and biochemical responses to I131 in this case however his prognosis remains guarded.