Thyroid dysfunction — both hypothyroidism and hyperthyroidism — causes hair loss, and thyroid disease is one of the most important investigations to exclude in any patient presenting with new or worsening diffuse hair shedding. Understanding the connection helps patients seek the right investigation rather than treating the symptom without addressing the cause.
Hypothyroidism (underactive thyroid) produces a characteristic pattern of diffuse hair thinning and shedding, along with fatigue, weight gain, cold intolerance, and dry skin. The hair loss in hypothyroidism can affect the entire scalp and also characteristically the outer third of the eyebrows — a clinical sign that is specifically associated with thyroid deficiency.
Hyperthyroidism (overactive thyroid) also causes hair loss — the mechanism differs but the result is similar diffuse thinning. The accompanying symptoms — weight loss, heat intolerance, palpitations, anxiety — usually make the diagnosis apparent from the history, but hair loss alone as the presenting symptom of mild hyperthyroidism is not uncommon.
Investigation: thyroid function tests (TSH and free T4) are a standard part of the workup for unexplained diffuse hair loss. TSH is the most sensitive screening test — a normal TSH makes significant thyroid dysfunction unlikely as a cause of the hair loss.
Treatment: correcting the thyroid dysfunction — with levothyroxine for hypothyroidism or appropriate treatment for hyperthyroidism — stops the ongoing cause of hair loss. Hair recovery then follows over three to six months as the follicles return to normal cycling. Treating hair loss without correcting underlying thyroid disease is ineffective and delays appropriate management.
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