Which thyroid hormone has the easiest placental transfer?

The thyroid hormone that has the easiest placental transfer is T4. This is mainly because T4 has higher levels in circulation compared to other thyroid hormones, and it is more lipid-soluble, allowing for easier diffusion across the placental barrier.

T3, while also significant as an active thyroid hormone, is typically less abundant in circulation compared to T4, and although it can cross the placenta, the transfer is not as efficient or prevalent as that of T4.

TSH and TRH are not thyroid hormones but pituitary hormones that regulate the production of T3 and T4. They have limited or no direct transfer capabilities across the placenta, as they do not engage in the same passive diffusion mechanism as the thyroid hormones do.

Overall, T4’s abundance and its properties favor its transfer across the placenta, making it the most readily available thyroid hormone for fetal development.