Thyroid hormones
Thyroid hormones are two hormones produced and released by the thyroid gland, namely triiodothyronine (T) and thyroxine (T). They are tyrosine-based hormones that are primarily responsible for regulation of metabolism. T and T are partially composed of iodine. A deficiency of iodine leads to decreased production of T and T, enlarges the thyroid tissue and will cause the disease known as simple goitre. The major form of thyroid hormone in the blood is thyroxine (T), which has a longer half-life than T. In humans, the ratio of T to T released into the blood is approximately 14:1. T is converted to the active T (three to four times more potent than T) within cells by deiodinases (5'-iodinase). These are further processed by decarboxylation and deiodination to produce iodothyronamine (Ta) and thyronamine (Ta). All three isoforms of the deiodinases are selenium-containing enzymes, thus dietary selenium is essential for T production. Edward Calvin Kendall was responsible for the isolation of thyroxine in 1915.
Kent Holtorf
Holtorf states that the standard thyroid tests which are typically relied upon by primary care and specialist physicians to diagnosis hypothyroidism (low thyroid) and determine dosage are insufficient because they focus on TSH levels as an indicator of thyroid function and miss a large percentage of people with low thyroid. This is discussed extensively on the website of his nonprofit organization the National Academy of Hypothyroidism. Instead, Holtorf advocates labs and diagnostics which look at all thyroid hormone levels, with a particular emphasis on free T3, triodothyronine, the active hormone both produced by the thyroid and converted from T4, levothyroxine, and reverse T3, a metabolite of T4 conversion usually created during times of stress or trauma. This is contrary to position statements by societies such as the American Thyroid Association and American Association of Clinical Endocrinologist. Holtorf condones treating hypothyroidism with compounded combinations of bioidentical thyroid hormones.