Thyroid hormones are essential for the normal metabolic functioning of all tissues in the body, and a wide array of symptoms are therefore associated with abnormalities in thyroid hormone production and activation. Even when apparently adequate amounts of thyroxine are produced by the thyroid gland, thyroid function is profoundly affected by anything that disrupts conversion of thyroxine (T4) to the active thyroid hormone, triiodothyronine (T3). This conversion takes place primarily at the cellular level within tissues — only 20% of circulating T3 is generated by conversion of T4 within the thyroid gland itself.
This is Part 1 of a two-part series. Read Part 2: The Role of Reverse T3 in Thyroid Assessment.
The Deiodinases: Gatekeepers to Intracellular Thyroid Hormone Bioavailability
Both circulating and intracellular thyroid hormone levels are controlled by a complex, highly regulated system involving three iodothyronine deiodinases (known as D1, D2 and D3), which selectively remove iodine atoms to create the different hormones. This system not only controls the relative amounts of both active and inactive hormone in the cell, but it also has the effect of conserving iodine, which has multiple intracellular functions.
What Is Reverse T3?
Reverse T3 (3,3’,5’-triiodothyronine, rT3) is a biologically inactive metabolite of thyroxine (T4) formed by selective deiodination. The active thyroid hormone T3 is formed by removal of an iodine atom in the outer ring of T4, while rT3 is formed by removal of an iodine atom in the inner ring of T4. Relative amounts of each are determined by the activity of the respective deiodinase enzymes, which are regulated by hormonal and nutritional factors and physiological conditions.
What Affects Circulating Levels of rT3 and T3?
Much experimental research into the deiodinase systems has necessarily been done in animals, and when studying the research on reverse T3 it is important to remember that humans are not the same as rodents. In normal adults, rT3 circulates at roughly 40 times the level of free T3 (reference range in normal adults for rT3 is about 9–25 ng/dL, while for free T3 it is 2.5–6.5 pg/mL). The relative levels of rT3 and T3 are affected by factors that stimulate or inhibit the activities of the deiodinases, as described below.
Type 1 Deiodinase (D1)
In humans, the D1 enzyme has a much higher substrate preference for rT3 than for T4, suggesting that its main function is to deiodinate rT3 to form T2, both conserving iodine and clearing rT3. In conditions in which D1 is downregulated, an increase in circulating rT3 is seen because the conversion of rT3 to T2 is significantly reduced. D1 is downregulated by selenium deficiency and in the non-thyroidal illness syndrome, as well as some cancers. It is also inhibited by some drugs, notably amiodarone, propranolol, propylthiouracil, dexamethasone and ipodate. On the other hand, D1 is upregulated by T3, which restores the clearance of rT3 and reduces the levels of circulating rT3 relative to T3.
Most studies find that successful treatment of the underlying illness, as well as maintaining nutritional support and re-introducing mobility as soon as possible, naturally restores the T3/rT3 ratio to normal.
Type 2 Deiodinase (D2)
Unlike D1 and D3 which are located in the plasma membrane of cells, D2 resides in the endoplasmic reticulum inside the cytosolic compartment, where the T3 generated by its activity can directly enter the nucleus and interact with thyroid receptors. The activity of D2 is therefore a primary determinant of intracellular T3 availability and thyroid receptor occupancy. Unlike D1, D2 activity is increased in non-thyroidal illness.
Type 3 Deiodinase (D3)
The D3 enzyme is not normally expressed in adult tissues, whereas it is present in the placenta and the neonate where it protects the developing organism from excess thyroid hormone activity. It is, however, reactivated in conditions associated with a reduced metabolic rate — particularly starvation, severe carbohydrate restriction, tissue injury, reduced tissue oxygen supply, critical illness, chronic inflammation and some cancers. D3 serves to inactivate T4 by converting it to rT3, limiting the amount of T4 that can be used to form T3 and thus slowing the metabolic rate.
Comprehensive Thyroid Testing at Hormone Lab UK
Because thyroid function depends on so much more than TSH alone — including Free T3, Free T4, TPO antibodies and iodine status — a comprehensive approach to testing is essential. Our Thyroid Test Kit (with Specialist Doctor Report) measures TSH, Free T4, Free T3 and TPO antibodies from a simple at-home blood spot collection, with full specialist interpretation included.
Because selenium and iodine are both critical cofactors for deiodinase activity and thyroid hormone production, our Thyroid & Iodine Test Kit adds urinary iodine measurement to the full thyroid panel — identifying a commonly missed nutritional driver of impaired T4-to-T3 conversion.
Because cortisol and inflammatory stress are among the most important drivers of D3 reactivation and impaired thyroid hormone conversion, our Thyroid & Cortisol Test Kit measures both systems together — providing a more complete picture of the factors driving persistent thyroid symptoms despite apparently normal T4 levels.
Originally by Margaret Groves, ZRT Laboratory. Reproduced with permission. Last reviewed: May 2026.