Reverse T3

Reverse T3 (rT3) is an inactive thyroid hormone metabolite produced when the body perceives an excess of thyroid hormone, primarily formed by removing the wrong iodine atom from thyroxine (T4). 1 Peat argued that rT3 is mostly caused by high cortisol and stress hormones, and that a supportive diet of fruit, orange juice, and milk can resolve the issue as effectively as taking small amounts of T3. 1 The molecule acts as a competitive antagonist, partly occupying the same response site that active T3 would, thereby blocking its metabolic effects. 3 Crucially, T3 itself cannot be converted into rT3; the inactive metabolite is made exclusively from T4. 2

The primary driver of rT3 accumulation is the stress-induced activation of specific deiodinase enzymes. Peat explained that high cortisol causes the liver to turn T4 into rT3, which interferes with active T3 function even more than T4 itself does. 5 Dinkov elaborated that any condition inhibiting liver function—such as increased fatty acid oxidation during fasting—leads to a buildup of T4, which the body then converts into rT3. 10 Estrogen also plays a key role by blocking the conversion of T4 to T3 in the liver, shunting it toward rT3 production instead; this explains why women under stress are particularly susceptible to this pathway. 2 Local enzymes can eliminate rT3, but when those are blocked by stress substances, rT3 accumulates and exacerbates a hypothyroid state. 3

Clinically, Peat observed that administering pure T3 can rapidly reverse the anti-metabolic effects of rT3. He described a patient in myxedema coma whose condition worsened as her T4 dose was escalated to 500 micrograms daily; she recovered immediately when switched to 25 micrograms of T3. 6 Small doses of T3 usually tend to normalize rT3 if it is high, because rT3 is made from T4 and T3 supplementation bypasses the problematic conversion step. 2 However, Peat warned that taking large boluses of T3 on an empty stomach creates a toxic excess perceived by the liver, which then upregulates deactivating enzymes and can lead to extreme hypothyroidism hours later. 4 He recommended nibbling small amounts of a T3 tablet throughout the day to avoid this counterproductive enzyme induction. 4

The broader metabolic context reveals that rT3 elevation reflects a catabolic state driven by low sugar availability and chronic stress. Peat noted that a low-sugar diet can cause chronically high cortisol, which induces the deiodinase that makes inactive rT3. 11 Fasting exacerbates this by breaking down muscle tissue, flooding the bloodstream with antithyroid amino acids like tryptophan, cysteine, and methionine that suppress thyroid output. 10 Even when supplementing T3, ongoing stress can still generate rT3 because cortisol release and estrogen production independently drive the inactivating enzyme pathway. 2 The condition reflects a vicious circle where stress, endotoxin, and estrogen lower carbon dioxide production and thyroid function, further impairing liver clearance and allowing more rT3 to accumulate. 9