Thyroid dysfunction is one of the most common medical causes of hair loss in both men and women, and it is significantly underdiagnosed because its symptoms overlap with many other conditions. Understanding the connection between thyroid health and hair is relevant for anyone experiencing hair loss that does not fit the typical androgenetic pattern.
Both hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid) can cause hair loss, though through different mechanisms. In hypothyroidism, reduced thyroid hormone slows all cellular processes, including the hair growth cycle. The result is diffuse thinning across the scalp, often accompanied by dry, coarse hair texture, fatigue, weight gain, and cold intolerance. Hair loss from hypothyroidism typically affects the outer third of the eyebrows as well as the scalp.
Hyperthyroidism causes hair loss through accelerated cellular turnover — the hair cycle speeds up, causing premature shedding before adequate length or density is reached. Patients with hyperthyroidism often describe fine, soft, very thin hair rather than the coarse diffuse thinning of hypothyroidism.
The critical point is that thyroid-related hair loss is reversible with appropriate treatment. Patients who have their thyroid condition adequately managed — either with thyroxine supplementation for hypothyroidism or antithyroid medications for hyperthyroidism — typically see hair regrowth over the following six to twelve months.
A simple thyroid function blood test (TSH and free T4) will reveal thyroid dysfunction. This test should be part of the baseline investigation for any patient presenting with diffuse, non-patterned hair loss.
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