Age-related decreases in the thyrotropin (TSH) responsiveness to thyrotropin-releasing-hormone (TRH) stimulation and to the inhibitory effect of triiodothyronine (T3); in vitro study on superfused rat pituitaries

Abstract The effect of age on the thyrotropic function was investigated in vitro by superfusing pituitary fragments obtained from 2–3-month- and 24-month-old male Wistar rats with medium 199 (GIBCO) and by measuring basal TSH secretion and TSH response to a 6-min pulse of TRH (10 nM) : a/ in the absence and b/ in the presence of T 3 (100 nM). TSH was measured by RIA in 3-min fractions with rat TSH materials from the NIADDK. The TRH-induced TSH release elicited by pituitary fragments from the old rats was decreased in comparison to that found in young animals. Addition of T 3 to the superfusion medium did not alter basal TSH release but significantly decreased the TSH secretory response to TRH in the young rats. This response was not modified in the old animals. Our results suggest that aging induces not only a TSH hyporesponsiveness to TRH stimulation but also a decrease of this responsiveness to the inhibitory effect of T 3 which could be related to a decreased TSH synthesis and to an age-related impairment of T 3 action on the thyrotrophs.