1. Many factors affect the rate of disappearance of thyroxine from the peripheral blood. The normal animal is exposed to these to differing extents throughout a 24‐hr day. Conflicting results have been described with regard to circadian variations in several parameters of thyroid physiology. 2. The disappearance of radioactive thyroxine from the blood has been followed in intact and thyroidectomized calves and thyroidectomized rats fed either at night or during the day. 3. When calves were fed during the day the rate of disappearance of thyroxine from the blood was significantly faster in the day than at night. When fed at night the nocturnal disappearance rate became faster than the diurnal rate. 4. The circadian rhythm persists in the thyroidectomized calf and is hence not solely a feature of hypothalamic, pituitary or thyroid function. 5. On a full diet (Oxo Ltd., diet 41 B) the faster disappearance of thyroxine from the blood occurs at night when rats feed at night and during the day when they feed during the day. 6. After bile‐duct ligation the faster disappearance still occurs in the period in which the animal is feeding. This occurs even if the diet is solely glucose. When feeding on pure glucose the effect is less marked. 7. The substitution of a glucose diet for a full oxoid diet (Oxo Ltd., diet 41 B) results in a fall of 33·5% in the over‐all disappearance rate in bile‐duct‐ligated animals. 8. These results show that feeding, and the processes associated with it, are important factors determining utilization of thyroxine. This is so even after the prevention of any biliary‐faecal loss of thyroxine.
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