Gestational age-related changes in plasma free thyroxine (FT 4 ) concentration in maternal and foetal goats were determined for 65-140 days of pregnancy. The placental permeability to maternal thyroxine (T 4 ) and iodine (I) during the last trimester of pregnancy was evaluated.The presence of T 4 in foetal circulation at 65 days' gestation, the absence of a significant correlation between maternal and foetal levels and the absence of any transplacental diffusion of 125 I-T 4 showed autonomous foetal thyroid function during pregnancy in goats. Significantly low maternal FT 4 was measured at 95 and 110 days' gestation (32-7 and 31 -5 pmol/1 respectively). Plasma concentration increased to 48 -6 pmol/1 at day 125 then fell before parturition to a level similar to that of non-pregnant goats (41-9 pmol/1). In contrast, foetal FT 4 increased with gestational age to 110 and 125 days when levels of 64-0 and 59-1 pmol/1 respectively were greater than those at all other ages, suggesting that the foetal hypothalamic-pituitary-thyroid is functioning at a higher rate than the maternal system around this time. This was supported by increased foetal thyroid affinity for I when compared with the maternal thyroid at 110-140 days' gestation.A significant cubic relationship between foetal FT 4 and gestational age was established.

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