T4 (Thyroxine)

T4 (thyroxine) is a precursor hormone that has no intrinsic metabolic activity until it is converted into the active hormone T3 (triiodothyronine). 2 Peat consistently argued that T4 is not the active thyroid hormone, and its function depends entirely on the liver's ability to remove one iodine atom in the correct position. 2 Dinkov has written that T4 "has no hormonal effects" and that prescribing it as monotherapy is "stupid" because it is merely a precursor. 11

The clinical failure of T4 monotherapy is rooted in the flawed testing that led to its commercialization. In the 1940s, drug companies synthesized thyroxine and tested it on healthy, 25-year-old male medical students, concluding it worked identically to natural desiccated thyroid. +5 Peat emphasized that this population had optimally functioning livers capable of efficient conversion, but the product was never tested on the population most affected by hypothyroidism: women. 2 Because estrogen antagonizes thyroid function and slows liver metabolism, women—especially those over 30—often cannot convert T4 to T3 effectively. 3 In these individuals, T4 can accumulate and act as an antithyroid agent, competitively displacing T3 from tissues and worsening hypothyroid symptoms. 3

Mechanistically, when the liver is sluggish due to hypothyroidism, high estrogen, or stress, T4 is shunted into the reverse T3 (rT3) pathway rather than being activated to T3. 2 Reverse T3 functions as a direct antagonist at the receptor, blocking the effects of active thyroid hormone and slowing metabolism. 11 Peat described cases where increasing T4 doses led to myxedema coma because the patient's body contained no detectable T3, a state immediately reversed by intravenous T3. 2 In brain tissue specifically, an excess of T4 that cannot be converted suppresses oxidative metabolism, as demonstrated in experiments where adding thyroxine to brain slices decreased oxygen consumption. 3 The brain normally maintains a 1:1 ratio of T4 to T3 by concentrating T3 from the serum, but flooding the system with T4 disrupts this equilibrium. 2

The conversion of T4 to T3 is heavily dependent on liver health and nutritional cofactors. Selenium is required for the local deiodinase enzymes that convert T4 to T3, and a deficiency will impair activation. 2 Prolonged stress, with elevated adrenalin and cortisol, promotes the conversion of T4 into reverse T3 rather than active T3. 3 Peat noted that hypothyroid individuals are often deficient in magnesium, and supplementing thyroid hormone can unmask a severe deficiency that causes intolerance. 3 Dinkov has added that because the half-life of T4 is approximately one week, daily dosing leads to accumulation that burdens an already compromised liver, potentially inducing fibrosis in tissues over time. 2 Roddy has cited evidence that T4 monotherapy can burden the liver and make patients feel progressively worse, referencing the experiences documented on forums like "Stop the Thyroid Madness." 5

Therapeutically, Peat recommended using a combination of T4 and T3 that mimics the physiological ratio secreted by the thyroid gland, or using T3 alone in frequent small doses. 2 He noted that while the standard synthetic combination uses a 4:1 ratio of T4 to T3, many people experience clearer thinking with a ratio closer to 3:1. 9 Because T3 has a short half-life and acts quickly—Peat described chewing a 5 mcg chip of Cytomel to restore memory within minutes—it should be taken multiple times per day. 2 T4, by contrast, accumulates and reaches a steady concentration only after about two weeks. 9 Peat was clear that monitoring therapy requires tracking pulse rate, body temperature, and symptoms, not merely TSH suppression, which can be driven to zero by T4 alone while the patient remains functionally hypothyroid. 2