Paradoxical response of thyrotropin to L-dopa and presence of dopaminergic receptors in a thyrotropin-secreting pituitary adenoma.

We studied the dopaminergic control of TSH secretion in a patient with hyperthyroidism due to a TSH-secreting pituitary adenoma. A 34-yr-old previously thyroidectomized woman had mild clinical hyperthyroidism and a diffuse goiter without exophthalmos. She complained of headaches and had bitemporal hemianopsia. Serum T4 and T3 by RIA were elevated, and TSH was 112 microU/ml (normal range, 1.1-7.2). The alpha-subunit to TSH molar ratio was 1.7 (normal, less than 1), as reported in other patients with tumoral TSH hypersecretion. After TRH, a marked increase in TSH occurred. There was no evidence of pituitary deficiency. Skull x-rays and computerized axial tomography revealed an intrasellar tumor with suprasellar extension. Selective transsphenoidal adenomectomy was performed, and a pituitary tumor was removed. The tumor was almost entirely composed of cells reactive with antihuman beta TSH serum by indirect immunofluorescence. A unique feature of this patient was the marked increase in TSH levels after L-dopa administration. To our knowledge, this paradoxical response has never been reported previously in such patients. Using [3H]domperidone as ligand, dopaminergic receptors were demonstrated in the membranes of the adenomatous thyrotroph cells. The reason for the paradoxical response of TSH to dopaminergic agents is not known.