Serum lipids and lipoproteins in hypothyroidism

Obese strain (OS) chickens hatched in the NIU vivarium were observed, primarily with respect to serum lipid and lipoprotein parameters, over a period of 36 weeks. The hypothyroid birds began to show differences in total serum lipids as compared to non-OS controls as early as 4 weeks post-hatching. All lipaemic chickens displayed lack (or deficiency) of thyroid tissue. Although all the thyroid deficient chickens showed increases in serum lipids, they were individually highly variable with respect to both quantity of serum lipid and time of onset of severe lipaemia. Six of twenty four OS birds became severely hyperlipaemic. One of these displayed a level of serum lipid amounting to 323 mg/ml. The increased serum lipid consisted primarily of triglyceride and phospholipid and was not accompanied by corresponding increases in apoproteins B or A-I. However, because of its high content of palmitic acid, oleic acid and phospholipid, the bulk of the serum lipid in OS birds appeared to be derived from the liver rather than directly from the diet. It is concluded that sufficient phospholipid was present in the plasma to form micelles, thus reducing requirements for association of solubilizing apoproteins. Livers were considerably enlarged in hyperlipaemic birds, possibly to compensate for decreases in lipid synthesizing enzymes per unit mass of tissue. However, results of gamma-irradiation of OS chickens (showing exacerbation of hyperlipaemia) indicated that hyperlipaemia in OS birds may be largely due to widespread failure of the body's cells to utilize foodstuffs and of adipose tissue to store excessive amounts of fatty acids as triglyceride.