Thyrotropic Hormone Secretion in Cattle after Prolonged Thyroxine Therapy and after Complete Withdrawal

Abstract Seven lactating cows of several breeds were injected with thyroxine daily 50% in excess of their experimentally determined thyroxine secretion rate (TSR) for a period of 11–12 wk, and in one Holstein cow for 21 wk after the animals had passed their peak lactation. The Holstein cow also received a supplemental thyroxine treatment of 100% over its TSR for a further period of 8 wk. On cessation of thyroxine administration, milk yield decreased rather precipitously in all the animals for 10–12 days. After this period, there was a small rise in milk yield in some animals for a few days before a plateau was established. I 131 uptake by the thyroid was determined every other day in all animals for 3 wk. Thyroid I 131 uptake commenced 12 days after cessation of thyroxine treatment and attained normality by 3 wk. The time at which the thyroid began to show avidity for iodine collection synchronized with the time at which no further decline in milk yield was noticed after thyroxine withdrawal. It is suggested that the precipitous decline in milk yield after thyroxine withdrawal is due to subnormal levels until endogenous thyroxine secretion is re-established.