[Inadequate TSH secretion. Clinical features, diagnostic criteria, and therapeutic possibilities].

Inadequate secretion of TSH (IST) is a disorder which is diagnosed more frequently and earlier after the introduction of new immunoassay techniques which can distinguish between normal and suppressed TSH levels. For diagnosis high or unsuppressed TSH in required in the presence of elevated levels of the thyroid hormones. Its etiology may be tumor (TSH secreting pituitary adenoma) or non tumoral due to pituitary or generalized resistance to the thyroid hormones. Differential diagnosis between both etiologies is not easy, and several tests have been proposed but are not always discriminatory. Five cases of IST are presented in whom the diagnostic, clinical and therapeutic criteria have been analyzed. 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 triiodothyronine (T3), 3,5-diiodo 4-(3'-iodine 4'-hydroxyphenoxi) phenylacetic acid (TRIAC) and with bromocriptine. 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 bromocriptine. The subunit-alpha/TSH quotient and TSH response to TRH were variable with no stimulation being observed with metoclopramide in any case. Upon the demonstration of unsuppressed circulating TSH in the presence of biochemical hyperthyroidism, IST should be suspected to avoid erroneous diagnosis and treatments. Differentiation between neoplastic and non neoplastic origin may be difficult since the biochemical and neuropharmacologic parameters are not always discriminatory.