Post concentrations of thyroxine in plasma of dairy cows.

Concentrations of thyroxine in blood plasma were measured by radioimmunoassay in 32 dairy cows postpartum and during estrous cycles. Mean concentrations of thyroxine declined from 1 to 2 days postpartum to 5 or 6 days postpartum. Concentrations of thyroxine remained low at 7 to 10 days postpartum and during the first estrous cycle. During subsequent estrous cycles concentrations of thyroxine were elevated to those 1 or 2 days postpartum.