1. Serum non-esterified bile acid profile was examined in patients with thyroid dysfunction. Sixteen hyperthyroid patients, six hypothyroid patients, nine patients taking thyroid or antithyroid drugs and 26 healthy controls were studied. The medicated patients were euthyroid when serum samples were collected. Bile acid concentration was determined by the simplified microassay method involving mass fragmentation spectrometry.

2. The sum of the concentrations of the individual bile acids was not significantly different among the four groups. However, the composition of bile acid reflected the thyroid function. The most prominent bile acid was deoxycholic acid in the hypothyroid patients and chenodeoxycholic acid in the hyperthyroid patients. The serum bile acid profile of medically treated patients was similar to that of normal cpntrols. The ratio of the sum of deoxycholic and cholic acid to that of lithocholic and chenodeoxycholic acid was found to be a good indicator of thyroid function, while the ratio of cholic acid to chenodeoxycholic acid correlated poorly with it.

3. The characteristic effect of thyroid hormone on the serum bile acid composition in man was the shift from the ‘family’ of cholic acid to that of chenodeoxycholic acid. This is in agreement with experimental results in the rat, and suggests a specific action of thyroid hormone on the hydroxylating enzymes involved in the conversion of cholesterol into bile acids.

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