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We studied on the potential change in TRH disappearance rate in the status of TSH refractoriness in vitro. TSH secretion from the rat anterior pituitaries progressively increased in the presence of TRH in doses between 0.2-20 ng/ml. In contrast, administration of TRH in a dose of 200 ng/ml decreased TSH secretion as compared to TRH in a dose of 20 ng/ml. Disappearance rate of 2 ng/ml TRH in incubation media increased as compared to that of 20 ng/ml TRH, but it was statistically similar between 20 and 200 ng/ml of TRH. Incubated TRH was indistinguishable from the authentic tripeptide on a HPLC. These data indicate that in vitro refractoriness of TSH response to TRH did not depend on the significant change in TRH disappearance rate.

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