Predominant T3 synthesis in the metastatic thyroid carcinoma in a patient with T3-toxicosis.

A metastatic mixed papillary and follicular thyroid carcinoma synthesizing predominantly triiodothyronine (T3) producing thyrotoxicosis is described. There was a concentration of 131I in numerous masses on the lung as well as a mass in the neck. BMR was +38%. Serum T4, T3-RU and T3 were 6.6 microgram/100 ml, 35.1% and 9.0 ng/ml, respectively. Serum TSH was not detectable and was not increased by the injection of 500 microgram of synthetic TRH. Decompressive surgery, performed while the patient was receiving no thyroid medication, yielded tumor tissue for chemical analysis. The mass measured 4.5 cm X 4.0 cm X 3.0 cm. The normal thyroid tissue was found to be atrophic. Sedimentation pattern of the soluble protein revealed a small peak at 18 S without a 27 S peak, suggesting an accelerated hydrolysis of thyroglobulin (Tg). Both the Tg content of the tumor and the degree of iodination of the Tg were low, 0.53% (based on wet weight of tissue) and 0.07%, respectively. DIT-I, MIT-I, T4-I, and T3-I in hydrolysate of the tumor tissue were 17.8%, 29.1%, 7.1%, and 41.7%, respectively. Therefore, the high T3 in the serum in this case seems to arise mainly from the cancer tissue. Possible mechanisms for this predominant T3 production are discussed.