Effects of Deficiency and Excess of Thyroid Hormone on the Distribution of Fatty Acids in Lipids of the Rat

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Abstract
Administration of excessive Lthyroxine to intact rats promoted increases in the hepatic concentrations of all fatty acids, largely through increases in the concentrations of phosphatides and free fatty acids; however, the increases of palmitate and oleate were disproportionately great. In plasma, the proportions of stearate and arachidonate increased, while the proportions of oleate and linoleate decreased. No effect on the composition of epididymal fat was observed. In the hypophysectomized rat, excessive L-thyroxine promoted increases in the hepatic concentrations of stearate and arachidonate, as in the intact rat, but the hepatic concentrations of oleate and linoleate decreased. Hypothyroid (thyroidectomized) rats had greater than normal concentrations of fatty acids of intermediate molecular weight in liver, although the hepatic concentration of total fatty acids remained approximately normal. These fatty acids of intermediate molecular weight were evident in the triglycerides, phosphatides, steryl esters and free fatty acid fraction. The concentration of arachidonate decreased below normal, and that of octadecatrienoate increased above normal. Greater than normal proportions of fatty acids of intermediate molecular weight were evident in epididymal fat and plasma, also. In addition, 4 unidentified substances—2 in plasma and 2 in epididymal fat—were present in abnormally great concentrations. The results imply that hypothyroidism and hyperthyroidism affect both the processes of metabolism and the processes of distribution of lipids in the rat. (Endocrinology75: 179, 1964)