THE EFFECT OF THYROTROPHIN RELEASING HORMONE ON PITUITARY AND THYROID FUNCTION IN PRE- AND POST-NATAL LAMBS

Thyrotrophin releasing hormone (TRH, 50 μg) was injected into chronically catheterized foetuses at various ages from 107 days gestation and into lambs up to 13 days post-partum. Changes in the concentrations of TSH, prolactin, T3and T4in plasma were measured by radioimmunoassay.The initial concentration of TSH (1.2 ± 0.2, sem, ng/ml) and its increase in response to TRH was similar at all foetal ages tested. However, both pre-injection concentrations of prolactin in plasma and the magnitude of the response to TRH increased with foetal age. The responses in lambs up to 13 days old were greater than those in the foetuses.The concentration of T4was 6.3 μg/100 ml plasma in a 107 day-old foetus This had doubled by 128 days but declined again towards parturition. After TRH injection plasma T4concentration increased in lambs, but showed little or no change in 113 and 137 day-old foetuses.The concentration of T3increased from 4–5 ng/100 ml plasma in 110–116 day-old foetuses to 40 ng/100 ml just prior to parturition. Values were high in the young lambs (362 ± 79 ng/ml) but declined with increasing age. There was no response to TRH up to 130 days gestation. A small response was obtained in a 137 day-old foetus while the concentration of T3rose from 37 to 90 ng/100 ml in the 144 day-old foetus.Injection of luteinizing hormone releasing hormone in quantities ranging from 25 to 150 μg caused no changes in circulating levels of TSH and prolactin.It is concluded that although the foetal pituitary can respond to TRH by at least 107 days gestation, its maturity in relation to prolactin synthesis and release develops gradually, but most rapidly over the last 10 days of gestation. In comparison, the pituitary-thyroid system, known to be functioning before 107 days, may be insensitive to hypothalamic control. The evidence suggests that there is increased maturity of the thyroid gland with increasing foetal age, the concomitant rise in thyroid hormones initiating a negative feedback response.