Severe thyrotoxicosis due to functioning pulmonary metastases of well-differentiated thyroid cancer

We report two cases of thyrotoxicosis resulting from hyperfunctioning lung metastases from differentiated thyroid cancer. In both patients, a simultaneous diagnosis of thyrotoxicosis and metastatic thyroid cancer was made, based on thyroid function tests as well as 131I whole-body scans showing low thyroid uptake of radioiodine and multiple foci of intense 131I uptake in the lungs. After total thyroidectomy (performed in Patient 2 only) and 131I therapy (cumulative dose of 12.3 GBq in Patient 1 and 9.6 GBq in Patient 2), there was a rapid clinical improvement with significant reduction of the pulmonary metastatic disease in both patients: Patient 1 became euthyroid, while Patient 2 became hypothyroid. Analysis of the 54 cases reported in the literature, including the 2 cases described here, shows this to be a very rare cause of thyrotoxicosis and one that can pose serious problems for both the diagnostic evaluation and choice of therapeutic strategy when compared with the much more common nonhyperfunctioning metastases from thyroid cancer. Lesser degrees of thyroid hormone secretion by differentiated thyroid cancer may be detected and exploited diagnostically by the chromatographic analysis of serum for endogenously labeled thyroid hormones after 131I administration.