Plasma free and total iodothyronine levels in hypophysectomized and intact lamb foetuses during the last third of gestation

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Abstract
The changes in plasma levels of thyroxine (T4), free thyroxine (free T4), triiodothyronine (T3), free triiodothyronine (free T3), and reverse triiodothyronine (rT3) were observed in intact or hypophysectomized lamb foetuses during the last third of gestation. Hypophysectomy was performed at two stages: 100 and 120 days of gestation. Cord blood was collected after Caesarean section. In intact foetuses T4 and free T4 levels increased significantly between day 100 and 120 of gestation, but decreased from day 132 to birth. T3 levels, low until day 132, rose abruptly during the last 12 days of gestation, as did the T3/free T4 ratio. Free T3 levels, relatively high on day 100, decreased until day 130 to rise during the last days of gestation. Reverse T3 levels, high since day 100, increased sharply until day 132 as did the reverse T3/free T4 ratio, and they declined. In hypophysectomized foetuses, undetectable or very low levels of T4, free T4 and rT3 were observed on day 120 and 144. T3 levels, although decreased by pituitary destruction, remained at 39.7% (120 days) and 21.4% (144 days) of the values recorded in intact foetuses. Free T3 levels, normally depressed at the end of gestation, were not affected by hypophysectomy on day 120. When hypophysectomy was performed later, the levels of T4, free T4 and rT3 were slightly increased on day 144 as compared with those of the other hypophysectomized foetuses. This study confirms the changes in the plasma iodothyronine levels observed previously in the lamb foetus. Moreover, it appears that T4 thyroid secretion depends almost exclusively on foetal pituitary function, and that if the sheep placenta is impermeable to T4 and rT3, a significant transfer of maternal T3 can occur.