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regulates · 5
←PCOS
The excess androgens of polycystic ovary syndrome (PCOS) thin the hair on the scalp while making facial and body hair coarser: one hormone, two opposite-looking effects. Hair loss together with irregular periods, acne or excess body hair is a reason to have your hormones checked.
Thyroid hormone influences the hair growth cycle. With an underactive thyroid (hypothyroidism), hair often thins all over and turns dry and brittle, usually alongside fatigue and feeling cold. This kind of hair loss shows up on a blood test and usually recovers with treatment, so it is worth ruling out early.
←Iron
Iron deficiency is one of the common triggers of telogen effluvium, in which many hairs enter the resting phase together and shed a few months later. This kind of shedding is usually reversible once the trigger is removed, so with diffuse shedding, having your ferritin checked is more useful than buying an anti-hair-loss shampoo.
A major stressor can push a large batch of hair follicles into the resting phase at once, but the hair only sheds about 2–3 months later. So heavy shedding now often traces back to something months earlier: an illness, surgery, or a stretch of intense pressure.
Biotin is a cofactor for several carboxylase enzymes, and true deficiency does cause hair loss. In people who are not deficient, extra biotin does not grow hair; it can instead interfere with lab tests and make results such as thyroid function or cardiac troponin falsely high or low.