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

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Abstract
Thyrotropin releasing hormone (TRH, 50 .mu.g) was injected into chronically catheterized fetuses at various ages from 107 days gestation and into lambs up to 13 days post-partum. Changes in the concentrations of TSH [thyrotropin], prolactin [PRL], T3 [triiodothyronine] and T4 [thyroxine] in plasma were measured by radioimmunoassay. The initial concentration of TSH (1.2 .+-. 0.2, mean .+-. SE, ng/ml) and its increase in response to TRH was similar at all fetal ages tested. However, both pre-injection concentrations of PRL in plasma and the magnitude of the response to TRH increased with fetal age. The responses in lambs up to 13 days old were greater than those in the fetuses. The concentration of T4 was 6.3 .mu.g/100 ml plasma in a 107 day old fetus. This had doubled by 128 days but declined again towards parturition. After TRH injection plasma T4 concentration increased in lambs, but showed little or no change in 113 and 137 day-old fetuses. The concentration of T3 increased from 4-5 ng/100 ml plasma in 110-116 day-old fetuses 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 fetus while the concentration of T3 rose from 37 to 90 ng/100 ml in the 144 day old fetus. Injection of luteinizing hormone releasing hormone [25-150 .mu.g] caused no changes in circulating levels of TSH and PRL. Although the fetal pituitary can respond to TRH by at least 107 days gestation, its maturity in relation to PRL 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. There is increased maturity of the thyroid gland with increasing fetal age, the concomitant rise in thyroid hormones initiating a negative feedback response.