In the lamb, plasma thyroxine concentration changes abruptly during birth. In the foetus, plasma concentration early on the last day of gestation is 6–8 μg thyroxine/100 ml (Nathanielsz, Comline, Silver & Thomas, 1973). Within the first hour after birth plasma thyroxine concentration rises to between 16 and 20 μg/100 ml. It then returns within 3–5 days of birth to levels found in the foetus and remains stable for at least a further 56 days (Nathanielsz, 1970). Since the conversion of thyroxine to tri-iodothyronine may be an important step in the mechanism whereby thyroxine acts on its target tissues, the plasma concentration of tri-iodothyronine during these rapid fluctuations of thyroxine is of interest.
Foetal plasma was obtained by the use of chronic indwelling catheters (Comline & Silver, 1972), and neonatal plasma was obtained by jugular vein puncture. Triiodothyronine was measured by radioimmunoassay using a modification of the method of Hüffner
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