The passage of thyroxine and triiodothyronine from mother to foetus in pregnant rabbits, with a note on the concentration of protein‐bound iodine in foetal serum

Abstract

Recent evidence suggests that in rabbits the passage of exogenous thyroxine from mother to foetus is very slow during the early stages of pregnancy. Hall & Myant (1956) showed that the foetal:maternal serum concentration ratio (F: M ratio) does not exceed 0x02 at 24 hr after an intravenous injection of radioactive thyroxine into the mother, when the injection is given before the 19th day. Since the thyroid of a foetal rabbit does not begin to make thyroid hormone until the 20th day (Jost, Morel & Marois, 1949), this suggested that the early stages of foetal development take place in the presence of a concentration of thyroxine much lower than that in the mother's serum. However, several points were left in doubt.After the 18th day, the F:M ratio was not measured at intervals of more than 3 hr from the injection. Nor were any observations made on the passage of triiodothyronine across the placenta; therefore we did not exclude the possibility that the foetus gets a supply of triiodothyronine from the mother before its own thyroid begins to function. We also pointed out that the placenta might not be as impermeable to endogenous thyroid hormone, at physiological concentrations, as it is to exogenous thyroxine. Finally, we suggested that the walls of the maternal blood vessels in the placenta constitute the main barrier to the passage of thyroxine from mother to foetus, but the evidence for this was far from conclusive. This paper describes experiments bearing on each of these questions. A few observations on the concentration of protein-bound iodine in the serum of rabbit foetuses are also included.

Bidirectional Thyroxine Exchange in Pregnant Sheep

“…Similar results were obtained in rabbits by Hall and M yant [14] and Myant [15]. Transfer seemed to increase with the progress of pregnancy and when T3 was used instead of T4.…”

Ontogeny of Plasma-Free Thyroxine and Triiodothyronine Concentrations during the Perinatal Period and Maternofetal Transfer of Thyroid Hormones in the Rabbit

“…Myant [23] had administered 131I-thyroxine or l31I-thyronine to the rabbit doe and observed an increase in radioactivity in fetal serum with a fetomaternal ratio of 1:40. From this study it remains unclear whether thy roid hormones crossed the placenta in an intact form since free iodine rea dily crosses the placenta [24].…”

Serum proteins, including thyroxine‐binding proteins, in maternal and foetal rabbits

“…The observations on maternal and foetal serum in vitro may help to explain the apparent increase in placental permeability to thyroid hormone observed during the last ten days of pregnancy (Myant, 1958). If the placenta is impermeable to TBP, but permeable to unbound thyroxine, an increase in the binding capacity of the foetal serum would tend to increase the foetal: maternal thyroxine concentration ratio at equilibrium.…”

“…If, however, the injection is made towards the end of pregnancy, at 24 hr after the injection the concentration of radioactive thyroxine in the foetal serum is found to have risen to nearly half the concentration in the maternal serum (Myant, 1958). These observations could be explained by supposing that in the early stages of pregnancy the foetal serum has less power to bind thyroxine than the mother's serum, and that as pregnancy advances, the binding power of the foetal serum increases.…”