Detailed definition
Thyroxine (T4) contains four iodine atoms and is the dominant hormone produced by the thyroid gland. It serves largely as a circulating reservoir, with target tissues converting T4 to the more active T3 as needed via deiodinase enzymes. Most circulating T4 is bound to thyroxine-binding globulin (TBG) and other carrier proteins; only the small unbound (free) fraction is biologically active. Free T4 measurement is paired with TSH to confirm hypothyroidism (low free T4, elevated TSH) or hyperthyroidism (elevated free T4, suppressed TSH). Levothyroxine (Synthroid, Levoxyl, others) is synthetic T4 used to replace deficient thyroid function. It is well absorbed, has a long half-life that supports once-daily dosing, and is titrated against TSH every 6–8 weeks until stable.
Why it matters in menopause
In menopause-age women, the combination of TSH and free T4 establishes whether thyroid disease is present alongside (or instead of) menopausal symptoms. A subclinically hypothyroid pattern (elevated TSH, normal free T4) is common in midlife and may need treatment, especially if symptoms are significant or TSH is above 10 mIU/L.