Start with what is actually happening

By pattern, speed and whether the scalp itself is affected. Pattern loss thins gradually in a defined distribution — the crown and hairline in men, the part line in women — while telogen effluvium sheds diffusely across the whole scalp two to three months after a trigger. Alopecia areata produces discrete round patches. Redness, scale, pain or shiny skin where hair used to be suggests an inflammatory or scarring cause and warrants prompt medical assessment.

Shedding in the order of a hundred hairs a day is normal, because at any moment a proportion of follicles are in the resting and shedding phases of the growth cycle. The number itself matters less than a change in the ratio: what indicates a problem is a sustained increase in the share of hairs shedding, or hairs returning progressively finer and shorter than those they replaced.

From both sides. Pattern hair loss is polygenic, meaning many genes contribute, and the common belief that it descends solely through the maternal line is an oversimplification of one gene on the X chromosome. Family history on either side raises likelihood, and its absence does not exclude it.

Significant physical or emotional stress can trigger telogen effluvium, in which an unusual number of follicles enter the resting phase simultaneously and shed together roughly two to three months later. The delay is why the cause is often missed. It is generally self-limiting once the trigger passes. Ordinary day-to-day stress is not an established cause of pattern hair loss.

Which tests are appropriate depends on your history and examination, and that decision belongs to the doctor ordering them. Tests commonly considered when diffuse shedding is being investigated include a full blood count, iron studies including ferritin, and thyroid function. Where signs of androgen excess are present, hormonal testing may be added. Normal results do not exclude pattern hair loss, which is diagnosed clinically.