Ray Peat said:“Thyroid hormone is necessary for respiration on the cellular level, and makes possible all higher biological functions. Without the metabolic efficiency which is promoted by thyroid hormone, life couldn’t get much beyond the single-cell stage. Without adequate thyroid, we become sluggish, clumsy, cold, anemic, and subject to infections, heart disease, headaches, cancer, and many other diseases, and seem to be prematurely aged, because none of our tissues can function normally. Besides providing the respiratory energy which is essential to life, thyroid hormones seem to stimulate and direct protein synthesis.
In hypothyroidism there is little stomach acid, and other digestive juices (and even intestinal movement) are inadequate, so gas and constipation are common. Foods aren’t assimilated well, so even on a seemingly adequate diet there is ‘internal malnutrition.’ Magnesium is poorly absorbed, and a magnesium deficiency can lead to irritability, blood clots, vascular spasms and angina pectoris, and many other problems. Heart attacks, hardening of the arteries, and both high and low blood pressure can be caused by hypothyroidism."