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A quantitative investigation was made of the enterohepatic circulation of endogenous thyroxine (T₄) in the rat. Control and cold-acclimated rats were maintained on a low iodine diet and water containing ¹²⁵I-labelled iodide of known specific activity for 4 weeks. Faeces were then collected over a 3-day period. Bile was collected by means of a pipette inserted into the ligated intestine and flow rate was determined. Blood was obtained from the vena cava. The total and organic ¹³¹I content of serum, faeces and bile were measured by several techniques. From these values and the known specific activity of the [¹²⁵I]iodide, the concentration of organic iodine in the various samples was calculated. In control and cold-acclimated rats, the bile: plasma organic iodide ratios were 1·02 and 1·38 respectively. Bile flow, and hence the rate of secretion of organic iodine in bile, was greatly increased in the cold rats. Nevertheless, in both cold and control rats, the amount of organic iodine excreted daily in the faeces invariably exceeded the amount secreted in bile.

In a second study in which rats were equilibrated with varying doses of ¹³¹I-labelled T₄ administered daily, it was found that with increasing amounts of T₄, serum T₄ concentration and the urinary excretion of hormonal iodine tended to level off. In contrast the ability of the hepatic—faecal system to clear T₄ appeared to be relatively unlimited with doses up to 20 µg T₄/100 g body weight/day.

It was concluded that, in the rat, there is no net reabsorption of the endogenous thyroid hormone secreted into the intestine in bile.