INADEQUATE TSH SECRETION - CLINICAL-FEATU RES, DIAGNOSTIC-CRITERIA AND THERAPEUTIC POSSIBILITIES

Inadequate secretion of TSH (IST) is a disorder which is diagnosed mor e frequently and earlier after the introduction of new immunoassay tec hniques which can distinguish between normal and suppressed TSH levels . For diagnosis high or unsupressed TSH in required in the presence of elevated levels of the thyroid hormones. Its etiology may be tumoral (TSH secreting pituitary adenoma) or non tumoral due to pituitary or g eneralized resistence to the thyroid hormones. Differential diagnosis between both etiologies is not easy, and several tests have been propo sed but are not always discriminatory. Five cases of IST are presented in whom the diagnostic, clinical and therapeutic criteria have been a nalyzed. The cases of neoplastic IST (patients n.o 3, 4, and 5) showed a loss in circadian rhythm of TSH and absence of suppression with tri iodothyronin (T3), 3.5-diiodo 4-(3'-iodine 4'-hydroxyphenoxi) phenylac etic acid (TRIAC) and with bromocryptine. The circadian rhythm of TSH was maintained in the non neoplastic IST (patients n.o 1 and 2) as was suppressed with T3, TRIAC and bromocryptine. The subunit-alpha/TSH qu otient and TSH response to TRH were variable with no stimulation being observed with methoclopramide in any case. Upon the demonstration of unsupressed circulating TSH in the presence of biochemical hyperthyroi dism, IST should be suspected to avoid erroneous diagnosis and treatme nts. Differentiation between neoplastic and non neoplastic origin may be difficult since the biochemical and neuropharmacologic parameters a re not always discriminatory.