Uptake and metabolism of thyroid hormones by the rat foetus in early pregnancy.

Pregnant rats were injected with trace amounts of [125I]thyroxine [( 125I]T4) or [125I]-tri-iodothyronine [( 125I]T3). Maternal blood, organs and foetuses were removed 1 and 4 h later, homogenized and tri-chloroacetic acid added to precipitate radiolabelled iodothyronines. An hour after injecting [125I]thyroxine, 9-10 day old foetuses contained 21% of the radiolabel found in the same weight of plasma whereas by 13-14 days they contained 0.6%. An hour after injection of [125I]tri-iodothyronine, 9-10 day old foetuses contained 54% of the radioactivity present in the same weight of plasma. When related to tissue protein, the radiolabelled iodothyronine content in 9-10 day old foetuses was comparable with that in maternal brain, heart or ovary. De-iodination of [125I]thyroxine in 9-10 day old foetoplacental units was equal to that in maternal liver but greater than that in brain or heart. It is concluded that substantial amounts of thyroxine and tri-iodothyronine enter the rat foetus in early pregnancy but thyroxine uptake may be minimal in later pregnancy.