Everyone sheds hair, but clinically relevant hair loss occurs when shedding clearly increases, density falls or bald areas develop.
Diffuse shedding can occur with telogen effluvium, for example after significant illness, fever, surgery, childbirth, rapid weight loss or other physiological stress. Iron deficiency, thyroid disease, medication and nutritional problems can sometimes be relevant.
Blood tests are not necessary for everyone, but can be useful when the history suggests a specific cause.
Slowly progressive thinning at the crown, hairline or central parting can fit androgenetic alopecia. Treatments may include minoxidil and, in selected men, finasteride.
Round bald patches, broken hairs, pain, marked scaling or signs of scarring require a different differential diagnosis and may need in-person dermatology.
A doctor can often use history, pattern and photographs to decide whether treatment, blood testing, watchful waiting or physical examination is the logical next step.