"Because the actions of T3 can be inhibited by many factors, including polyunsaturated fatty acids, reverse T3, and excess thyroxine, the absolute level of T3 can't be used by itself for diagnosis. “Free T3” or “free T4” is a laboratory concept, and the biological activity of T3 doesn't necessarily correspond to its “freedom” in the test. T3 bound to its transport proteins can be demonstrated to enter cells, mitochondria, and nuclei. Transthyretin, which carries both vitamin A and thyroid hormones, is sharply decreased by stress, and should probably be regularly measured as part of the thyroid examination.
When T3 is metabolically active, lactic acid won't be produced unnecessarily, so the measurement of lactate in the blood is a useful test for interpreting thyroid function. Cholesterol is used rapidly under the influence of T3, and ever since the 1930s it has been clear that serum cholesterol rises in hypothyroidism, and is very useful diagnostically. Sodium, magnesium, calcium, potassium, creatinine, albumin, glucose, and other components of the serum are regulated by the thyroid hormones, and can be used along with the various functional tests for evaluating thyroid function."
Preventing and treating cancer with progesterone.
Read Ray Peat's articles on thyroid. The one I linked before is particularly useful in reference to how standard medical procedures conduct thyroid checks. Hypothyroidism is a common cause of hair loss in women.