Studies on plasma
triglycerides (TG) were performed in 10 nonobese and 16 obese patients with
hypothyroidism and in 13 with
hyperthyroidism. Nonobese, hypothyroid patients generally had normal levels of TG, but obese patients often had
hypertriglyceridemia. In most hypothyroid patients 1-thyroxine treatment lowered plasma TG, and most
hyperthyroid patients had low TG. One mechanism whereby
thyroid hormones might decrease plasma TG could be to increase
lipoprotein lipase (LPL). However, post-
heparin LPL was not increased after
therapy, nor was it increased in
hyperthyroid patients. In contrast, hypothyroid patients had abnormally low levels of post-
heparin hepatic
triglyceride lipase. In hypothyroid patients without
hypertriglyceridemia, clearance of
chylomicrons was normal. A few obese, hypothyroid patients with fasting
hypertriglyceridemia had low clearance of
chylomicrons, which may have been due in part to competition for removal of excess endogenous TG. Thus, no evidence was obtained for a significant abnormality in
chylomicron metabolism in
hypothyroidism. Nonobese, hypothyroid patients had normal synthesis and clearance of
very low density lipoprotein (VLDL)-TG. In contrast, VLDL-TG synthesis was increased in 8 obese, hypothyroid patients, and fractional clearance rates were relatively low compared to obese, euthyroid subjects. In striking contrast,
hyperthyroid patients had remarkable facility in clearing VLDL-TG. Thus, TG metabolism is not grossly deranged in
hypothyroidism, but
thyroid hormones apparently can promote catabolism of VLDL.