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	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 documented cases where increasing T4 doses made patients more hypothyroid, depressed, or even comatose—only reversing when T3 was given directly. He generally recommends combination T4/T3 products (like Armour, Cynoplus) at a 4:1 ratio rather than T4 alone.

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