Thyroid dysfunction — both hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid) — is a recognised cause of diffuse hair loss that can significantly affect hair density and texture. For patients experiencing unexplained hair loss, thyroid function testing is an essential part of the initial investigation.
In hypothyroidism, the reduced metabolic rate affects all rapidly dividing cells — including those in the hair follicle. Hair growth slows, the follicle fails to progress efficiently through the growth cycle, and diffuse thinning results. Hair often becomes coarser, drier, and more brittle in addition to the loss of density. The outer third of the eyebrow — a classic sign — may also thin.
In hyperthyroidism, hair shedding occurs through a different mechanism — acceleration of the hair cycle that leads to premature shedding and reduced follicle density. The hair often becomes finer and silkier in texture before shedding becomes noticeable.
Both conditions are diagnosable with a simple blood test (TSH and, where appropriate, T3/T4) and treatable with medication. The important message for hair loss patients is that thyroid-related hair loss is fully reversible once the underlying thyroid condition is properly managed. It may take six to twelve months after thyroid function is normalised for hair density to recover fully, but recovery is the expected outcome.
If you are experiencing diffuse hair loss without a clear cause — particularly if accompanied by fatigue, weight change, temperature intolerance, or mood changes — a thyroid function test is one of the first investigations to request from your GP.
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