A case of refetoff syndrome: Selective venous sampling for TSH is useful in differentiating thyroid hormone resistance from TSH secreting tumor.

Abstract

A 22-year old man with a goiter and clinical manifestations of mild thyrotoxicosis (finger tremor, palpitation, tachycardia) was diagnosed as a syndrome of inappropriate secretion of TSH. Serum concentrations of T4, free T4i T3 and TSH were 24.1,ug/100 ml, 4.07 ng/100 ml, 261 ng/100 ml and 1.12 ,uU/ml, respectively. Thyroidal 1311 uptake at 24 hr was 80%. The BMR was within the normal range. He had a normal TSH response to TRH (500,ug) with a peak level of 23.8pU/ml. The basal level of alpha-subunit of TSH was not elevated (0.35 ng/ml). Oral 1-T3 administration (75 and 150 ,cg daily) raised serum T3 concentration, reduced basal TSH and blunted TSH response to TRH. The diurnal variation of TSH was maintained. There was no evidence of abnormalities in the secretion of other pituitary hormones. These findings were compatible with thyroid hormone resistance. However, the presence of a microadenoma in the pituitary gland was suspected with CT scan. Bilateral and simultaneous venous sampling for TSH from inferior petrosal sinus showed no gradient in TSH concentration indicating that a TSH secreting pituitary tumor was unlikely. These data suggest that inappropriate TSH secretion in the present patient is resulted from resistance to thyroid hormone. In the present study selective venous sampling is useful to differentiate the thyroid hormone resistance from a TSH secreting tumor.thyroid hormone resistance ; venous sampling ; TSH secreting tumor Inappropriate hypersecretion of pituitary thyrotropin (TSH) is a rare disorder resulting from resistance to thyroid hormone or a TSH secreting tumor. This syndrome is characterized by elevation of serum thyroid hormones and a high or normal level of TSH. Basal levels of TSH and a-subunit and their response to