I wrote this essay to help myself understand why I wasn't having any luck optimizing my thyroid hormones using standard thyroid tests. Hope it helps you too! Read the full piece here: https://9gurus.com/how-deiodinase-e...0i8v0UfqQStXOd09i8KN_VhS2e2eHAzFGZgDCJaM_qYyg
Metabolism is largely dependent on adequate levels of thyroid hormone to create the energy needed by the body to carry out vital functions. Insufficient amounts of thyroid hormone can have broad, non-specific but wide-ranging effects. Such common symptoms of fatigue, brain fog, and weight gain are often associated with hypothyroidism, a condition where not enough thyroid hormone is produced to meet the metabolic needs of the body.
Upon suspicion of low thyroid function, a conventional endocrinologist will typically order a laboratory blood test to measure the amount of thyroid stimulating hormone, or TSH, that is present. TSH is a pituitary hormone that signals the thyroid gland to produce additional thyroid hormone. This is part of a signaling cascade that starts with the production of thyroid releasing hormone (TRH) in the hypothalamus, which then stimulates TSH and ultimately the thyroid gland itself.
Hypothalamus (TRH) -> Pituitary (TSH) -> Thyroid gland (T4 +T3) ->Cell
If the TSH number is “normal” by the typical laboratory range, most conventional doctors will stop investigations at this point. This leaves many people frustrated and confused, still suffering with clear symptoms of hypothyroidism, but without a diagnosis or treatment. That is because most conventional doctors only consider one type of hypothyroidism — as a result of insufficient secretion of thyroid hormone from the thyroid gland itself. If the TSH is normal, the reasoning goes, thyroid hormone production must be sufficient, otherwise, the TSH level would rise. Yet this TSH feedback loop is only one of many ways that thyroid problems can occur, and it is not even the most common source of thyroid problems, especially in those with chronic illness.
Functional medicine practitioners go a step further and will often diagnose hypothyroidism after testing the TSH, but also testing the free thyroid hormone levels, free T4and free T3directly in the blood. When these are found to be low or low-normal in the typical laboratory range, subclinical hypothyroidism may be diagnosed, and thyroid hormone prescribed. This (controversial) method typically improves on that used by the conventional endocrinologist, but still leaves many thyroid patients feeling less than optimal and wondering why.
The answer to that question depends on another aspect of thyroid physiology often overlooked during a clinical thyroid assessment. Blood levels of thyroid hormone are used as a proxy for thyroid function, but these are an indirect measure which may not (and often does not) truly reflect what is happening inside the cell when and where the thyroid hormone binds to the nuclear thyroid receptor and exerts its effects.
Thyroid hormones are not just controlled by the pituitary gland sending TSH to the thyroid gland, but are activated or inactivated locally, at the cell, by a group of enzymes called deiodinases. This local control by deiodinases allows the cell to adjust the amount of thyroid hormone based on the needs of the cell at any given time. The function of deiodinase enzymes varies for every individual based on genetic and environmental factors, which leads to the large variation in treatment response seen clinically to thyroid hormone medication.