Iodine metabolism and the effect of TSH in thyroid glands of early bovine embryos.

Abstract

The ontogeny of the thyrotrophin-thyroid axis during the first trimester was studied in 104 bovine embryonic thyroids taken from foetuses of crown-rump length 1.4 to 19.2 cm (25-120 days). The uptake of labelled iodine in vitro in the absence or presence of TSH was measured. The per cent incorporation of radioiodine into iodotyrosines and iodothyronines in the presence and absence of TSH was also studied. It was found that the foetal tissue displayed radioiodine uptake by 25 days of foetal life and the uptake increased with age. TSH caused a further increase in radioiodine uptake in foetuses of 40 days or older. Incorporation of radioiodine into MIT and DIT was apparent at 25 days and into T3 and T4 by 40 days of foetal life. Addition of TSH increased the proportion of total radioiodine found as DIT and thyroxine in foetuses of 40 days older. This TSH stimulation of radioiodine incorporation increased with age. However, the proportion of radioiodine found as MIT and T3 was not affected before 120 days of foetal life. This was in marked contrast to the adult thyroid where the proportion of radioiodine found as T3 was increased by the addition of TSH. It is concluded that the foetal thyroid can respond to TSH by at least 40 days of foetal life and that this response differs from that seen in the adult.