A healthy thyroid produces T4, T3, T2, T1 and Calcitonin; the two most important hormones being T4 and T3. Another, frequently overlooked, thyroid hormone is reverse T3, or rt3, which is an inactive mirror-image of the active hormone T3. Too much of the inactive isomer will result in hypothyroid symptoms.
Thyroid Hormone Production
T4 is the main hormone produced by the thyroid gland and it is relatively inactive. To be utilized T4 must undergo a conversion into T3, which is five times more active. The conversion takes place through an iodine atom being removed; rT3 is made when the “wrong” atom is removed.
Reverse T3 (reverse triiodothyronine) is the third-most common iodothyronine that the thyroid gland releases into the bloodstream. However, 95 percent of rT3 in the bloodstream was made elsewhere in the body as enzymes remove a particular iodine atom from T4. Most endocrinologists erroneously believe rT3 is just an inactive metabolite with no physiologic effect. Belief of this falsehood results in a misdiagnosed or undiagnosed condition.
Most endocrinologists and physicians measure thyroid dysfunction by the body’s TSH and T4 levels. If T4 levels are low, TSH is increased indicating a low functioning thyroid. The problem with these standard tests is they fail to measure whether T4 is being properly converted to T3 or if more is being converted to rT3 in the bloodstream causing “tissue hypothyroidism”.
It is normal to have a certain level of rT3 in the bloodstream, but too much will block T3 receptors severely impairing the synthesis of active T3 and sabotaging metabolic functioning on a cellular level. This issue is exacerbated when the body undergoes stress. Stress causes the body to produce higher levels of cortisol as a way of conserving energy. This will further inhibit T4 to T3 conversion resulting in a higher T4 to rT3 conversion causing a competition between rT3 and T3 at the receptor sites. Inactive rT3 will dock itself to the receptor sites blocking them and preventing active T3 from getting in, sabotaging normal metabolic functioning.
Strong Thyroid Inhibitor
Studies have demonstrated that rT3 is an extremely potent inhibitor of T4 to T3 conversion. When compared to Propylthiouracil (PTU), a crystalline compound used to treat hyperthyroidism, rT3 demonstrated to be 100 times more potent in reducing conversion. If you’ve undergone standard thyroid tests, but are still experiencing hypothyroid symptoms such as cold hands/feet, dry skin, brittle nails, weight gain/difficulty losing weight, etc., it may be time to have a full thyroid panel ran, complete with rT3.