Does a Diurnal Rhythm of Thyroid Activity Exist in Dairy Cattle?

Abstract
The diurnal rhythm of thyroid activity in Holstein cows was studied using 2 parameters of the thyroid activity. The thyroxine I131 disappearance rate and plasma PBI (protein bount iodine) data indicated the absence of any diurnal rhythm effects on thyroid activity under controlled conditions. The results suggested the validity of the daily thyrozine I131 disappearance rate in estimating the daily thyroxine secretion rate and the validity of the PBI determination of blood collected at any time during the day or night.