>Although cholesterol is protective against oxidative and cytolytic damage, the chronic free radical exposure will oxidize it. During the low cholesterol turnover of hypothyroidism, the oxidized variants of cholesterol will accumulate, so cholesterol loses its protective functions.
>In the healthy organism, cholesterol is constantly being synthesized, and constantly converted into steroid hormones, and, in the liver, into the bile salts that are secreted to emulsify fats in the intestine. Thyroid hormone and vitamin A are used in the process of converting cholesterol into pregnenolone, the immediate precursor of progesterone and DHEA. Anything that interfered with these processes would be disastrous for the organism. The supply of cholesterol, thyroid and vitamin A must always be adequate for the production of steroid hormones and bile salts. When stress suppresses thyroid activity, increased cholesterol probably compensates to some extent by permitting more progesterone to be synthesized.
>In very young people, the metabolic rate is very high, and the rapid conversion of cholesterol into pregnenolone, DHEA, and progesterone usually keeps the level of cholesterol in the blood low. In the 1930s, a rise in the concentration of cholesterol was considered to be one of the most reliable ways to diagnose hypothyroidism (1936 Yearbook of Neurology, Psychiatry, and Endocrinology, E.L. Sevringhaus, editor, Chicago, p. 533). With aging, the metabolic rate declines, and the increase of cholesterol with aging is probably a spontaneous regulatory process, supporting the synthesis of the protective steroids, especially the neurosteroids in the brain and retina.