Thyroid Hormone-Induced Dissociation between Plasma Triglyceride and Cholesterol Regulation in the Rat*

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Abstract
Exposure to thyroid hormone (T4) has been known to affect the plasma triglyceride (TG) concentration as well as the plasma cholesterol level, but the mechanism (s) and reproducibility of the TG response have not been defined. In the present study, we examined TG secretion in vivo in rats maintained in hypothyroid, euthyroid, and hyperthyroid states for 6 weeks. Endogenous TG secretion, as evaluated by the Triton methodology, increased in a linear fashion in relation to increasing plasma T4 concentration and was correlated with progressively increased levels of plasma TG. The largest response was seen in the transition between the hypothyroid and euthyroid states. Plasma total cholesterol concentration was progressively reduced with increasing thyroid hormone exposure, demonstrating a change the opposite of that observed for plasma TG. The plasma high density lipoprotein cholesterol concentration was reduced in the transition between the hypothyroid and euthyroid states, but a further reduction in hyperthyroid animals was not observed. The results suggest that very low density lipoprotein TG metabolism is altered at the site of secretion with increasing exposure to T4. The possible relationship between TG secretion and alterations in FFA availability, insulin secretion, and starvation is discussed. It would thus appear that the thyroid-induced changes in lipid physiology result in a divergence of plasma cholesterol and TG concentrations.

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