A case of the syndrome of inappropriate secretion of TSH

A girl aged 4 years with goiter and accelerated physical and skeletal growth was found to be hyperthyroid on the basis of elevated serum thyroid hormone level, nevertheless both the basal TSH and TSH responsiveness to TRH were maintained within the normal range. Serum TSH was suppressed by exogenous T3 and dexamethasone administration, but not significantly changed after propylthiouracil (PTU) treatment. The diurnal rhythmicity of anterior pituitary hormones was preserved with the high nocturnal peak of TSH and prolactin. Clinically, neither thyrotoxic signs nor evidences of pituitary tumor were observed. Her accelerated growth and elevated thyroid hormone level appeared to be induced by inappropriate secretion of TSH. In view of the literature, this is the first case of the syndrome of inappropriate secretion of TSH excluding the neoplastic origin in Japan.