Why hair changes during periods of change

Hair follicles cycle between a growth phase and a resting phase. A sudden change in the body, including rapid weight loss, can shift more follicles into the resting phase at once, with shedding following two to three months later. Clinicians call this telogen effluvium.[1]

Recent reviews of people taking GLP-1 medication (semaglutide, tirzepatide) report hair loss more often than in people who aren't, with telogen effluvium the most common form and women affected more than men.[2][3] Rapid weight loss and lower intake of nutrients such as iron, zinc and vitamin D are the proposed drivers.[3][4]

People with diffuse shedding are often found to have low ferritin (iron stores), low vitamin D and, less often, low zinc when tested. In one series, 45% had low ferritin and 34% low vitamin D.[5] A meta-analysis of more than 10,000 women found lower ferritin in those with non-scarring hair loss than in those without.[6] Vitamin D levels are also lower in telogen effluvium patients than in controls.[7]

NHS prescribing guidance for tirzepatide notes that micronutrient supplements may be needed where dietary intake is insufficient.[8]

During pregnancy, higher oestrogen holds more hairs in the growth phase. After birth, oestrogen falls and those hairs move into the resting phase together. In the largest description of the pattern, shedding began on average 2.9 months after birth, peaked at 5.1 months and settled by about 8 months.[9] It usually resolves on its own.

The postpartum months also follow blood loss at delivery, and iron deficiency without anaemia is common and often untested in new mothers.[6][10] Shedding that continues past nine months, is patchy, or comes with a sore scalp is worth a GP appointment, because postpartum shedding can unmask other patterns of hair loss.[10][11]

From the early forties, falling and fluctuating oestrogen shortens the growth phase of hair. Periods often become heavier before they stop, which lowers iron stores at the same time. Thyroid disease is common in the same age group and causes similar symptoms, so it's worth ruling out with a GP.

Patchy loss, a sore or scaly scalp, loss of eyebrows or body hair, or shedding that hasn't settled after six to nine months. Not all hair changes are nutritional.