25 euthyroidal patients with elevated serum cholesterol levels were treated with 4 mg of D-thyroxine daily for 4 weeks; controls were made after this period and after a withdrawal period of another 4 weeks. At the end of the treatment period there was a significant decrease of the serum cholesterol level, of the basal and TRH-stimulated serum TSH and of the thyroidal radioiodine uptake. The radioimmunologically determined values of total T 3 and Free Thyroxine index and of the serum iodine level rose significantly without being accompanied by clinical hyperthyroid symptoms. These changes can be explained on one hand by the well known cholesterol lowering effect of D T-4 and by the inclusion of D-isomeres of thyroid hormones in the radioimmunoassay, and, on the other hand, by the fact that the above-mentioned medication causes an uptake of iodine, and, possibly, by a direct inhibitory effect of D T-4 on the pituitary gland.