The circulating levels of growth hormone and thyroxine during the last 2 weeks of gestation, and the effect of thyrotrophin releasing hormone on their secretion, has been examined in 13 chronically catheterised pig foetuses. Porcine growth hormone, measured by homologous radioimmunoassay, in the foetal plasma was found to average 83.5 \m=+-\10.3 ng/ml, while thyroxine levels were 92.9 \m=+-\4.1nmol/l. There was no change in thyroxine or growth hormone levels with gestational age. Injection of 5 \ g=m\ g TRH produced a marked increase in plasma growth hormone levels (+ 194 \m=+-\51 ng/ml) but no change in thyroxine was observed. Injection of saline and subsequent blood sampling had no effect on the levels of either growth hormone or thyroxine.Since growth is more rapid in utero than at any other time during the life of an animal, it seems likely that there would be some hormonal control of cell division in the foetus. Growth hormone, perhaps working through the somatomedins, is a likely canditate for this important role. It is clear that growth hormone is centrally involved in regu¬ lating growth in the post-natal animal even though its actions may be mediated and influenced by many other hormones. The role of growth hor¬ mone in stimulating growth in utero is, however, far from clear. Conflicting evidence is assessed differently by different authors. Thus Jost (1954, 1979) concludes that 'the pituitary gland has at most a slight effect on foetal (rat) growth' and
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