Thyroxine

Thyroxine
Abbreviation	T4
Molecular formula	C₁₅H₁₁I₄NO₄
Type	Prohormone (inactive thyroid hormone precursor)
Administration	Oral (tablets), occasionally topical
Bioavailability	~80% orally (well absorbed, but effectiveness depends heavily on liver conversion)
Synonyms	Thyroxine, Levothyroxine, L-T4, Synthroid, Levoxyl, Tirosint
Source	Endogenous: Thyroid gland; Supplemental: synthetic pharmaceutical or desiccated thyroid gland
Ray's verdict	Depends on context Cautious/Conditional: T4 alone is problematic for many people. It has no direct metabolic activity until converted to T3 by the liver. In the 1940s, drug companies synthesized thyroxine and tested it on young male medical students; it worked well because young men have excellent liver function. However, women (especially those with high estrogen) often have sluggish livers that cannot convert T4 to T3. In stressed individuals, excess T4 converts to reverse T3 instead of active T3, which can paradoxically worsen hypothyroid symptoms. Ray Peat documented cases where increasing T4 doses made patients more hypothyroid, depressed, or even comatose, reversing only when T3 was given directly. Combination T4/T3 products (like Armour, Cynoplus) at a 4:1 ratio are generally recommended over T4 alone.

Thyroxine (T4, levothyroxine) is the main hormone the thyroid gland secretes. It is a prohormone: most activity requires conversion to T3 in the liver and other tissues.

The gland releases roughly 80% T4 and 20% T3; most circulating T3 is made from T4 outside the thyroid. T4 redirects here.

History

Etymology

The name combines "thyroid" (see Thyroid hormone) with "oxy-" (for its oxygen-containing structure) and the amine suffix "-ine."

Edward Kendall isolated thyroxine in crystalline form in 1914, the first thyroid hormone to be identified, though its exact structure and the more active hormone T3 weren't characterized until decades later.

Thyroxine was isolated from thyroid tissue in the early 20th century. Synthetic levothyroxine entered routine use in the 1940s and largely replaced desiccated thyroid in mainstream endocrinology. See NDT for the glandular form.

Structure/Chemical properties

T4 is L-thyroxine (C₁₅H₁₁I₄NO₄): a tyrosine backbone with four iodine atoms. In serum it binds to thyroxine-binding globulin and albumin; only a small free fraction is available for cell uptake and deiodination.

Function/Mechanism of Action

Deiodinases in liver, gut, and peripheral tissues convert T4 to active T3. Reverse T3 (rT3) is an inactive route when conversion is shunted under stress or illness.

An excess of thyroxine, in a tissue that doesn't convert it rapidly to T3, has an antithyroid action.

— Ray Peat

In one experiment, thyroxine added to brain slices suppressed respiration, while T3 supports oxidative metabolism. Poor liver conversion, high Estrogen, and stress are common themes when T4-only therapy feels inadequate. See Hypothyroidism, Liver.

Medical uses/Effects

Levothyroxine (synthetic T4) is standard prescription therapy for hypothyroidism worldwide. Mainstream care titrates dose by TSH. Broda Barnes and Ray Peat called for temperature, pulse, cholesterol, and symptoms alongside or instead of isolated TSH rules.

Oral T4 absorption depends on adequate stomach acid; Helicobacter pylori infection lowers stomach acid and can impair it. In a study of patients who stayed hypothyroid despite high-dose T4, eradicating H. pylori dropped mean TSH from 30.5 to 4.2 mIU/mL and raised free T3 and free T4 into normal range, with 21% of cases overshooting into transient thyrotoxicosis as absorption normalized. Persistently high TSH despite a high T4 dose is a signal to check for H. pylori or other malabsorption rather than simply raising the dose further; this mechanism is specific to oral T4 (and NDT, which is mostly T4) since direct T3 bypasses the stomach-acid-dependent absorption step entirely. Vitamin C both helps prevent H. pylori infection and improves eradication-therapy success rates.

Dosing

Supplement protocols, warnings, labs, and titration are on Roadmap/05 - Supplementing T3/T4. See also NDT, Triiodothyronine, Basal temperature, and Harm reduction.

Side/Adverse effects

Overtreatment causes hyperthyroid symptoms (tachycardia, anxiety, weight loss). Excess T4 without adequate conversion to T3 can worsen hypothyroid symptoms, including depression and low energy, until active T3 is supplied.