Thyroid hormones
The thyroid hormones function via a well-studied set of nuclear receptors, termed the thyroid hormone receptors. These receptors, together with corepressor molecules, bind DNA regions called thyroid hormone response elements (TREs) near genes. This receptor-corepressor-DNA complex can block gene transcription. Triiodothyronine (T3), which is the active form of thyroxine(T4), goes on to bind to receptors. The deiodinase catalyzed reaction removes an iodine atom from the 5' position of the outer aromatic ring of Thyroxine's (T4) structure. When triiodothyronine (T3) binds a receptor, it induces a conformational change in the receptor, displacing the corepressor from the complex. This leads to recruitment of coactivator proteins and RNA polymerase, activating transcription of the gene.
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.