Metabolism of Thyroxine in Hyperthyroidism in Cattle.

Abstract

When dairy cows were maintained in a hyperthyroid condition by injection of T4 50% above their winter TSR (thyroid secretion rate), it was observed that rate of disappearance of thyroxine-I131 from blood was doubled. Increasing T4 injection to 100% above winter TSR further decreased t1/2. Milk production in all cows at 50% above normal level of T4 increased 20% or more and the increase was sustained for many weeks. Increase to 100% above normal level of T4 had little or no further influence upon lactation.