In brief, thyroid dysfunction causes diffuse hair loss across the entire scalp and affects up to 50% of patients with untreated thyroid disorders. A 2025 study found both TSH and Free T4 were significantly lower in women with hair loss compared to controls. Yet both values were within the normal reference range. TSH-only testing misses the T3 conversion problem that many thyroid hair loss cases involve, and if you take biotin supplements, you need to stop them three, five days before any thyroid blood test, or your results may be falsely reassuring. Thyroid hair loss is a form of diffuse hair shedding caused by abnormal levels of thyroid hormones, T3 and T4, that disrupt the hair growth cycle, affecting up to 50% of patients with untreated thyroid disorders. Both hypothyroidism, underactive, and hyperthyroidism, overactive, can trigger hair loss. Hypothyroidism is more common and more often missed because it develops gradually, and its early signs are easily attributed to other causes. A study published in diseases in October 2025 enrolled 100 women presenting with hair loss and 100 age-matched healthy controls, and measured a comprehensive panel including TSH, free T4, ferritin, zinc, vitamin D, and B12. The mean serum TSH level in the hair loss group was significantly lower than in the control group, 1.74 vs 2.35 UIU ML, P 0.001. The mean free T4 level was also significantly reduced in the hair loss group vs controls, 1.11 vs 1.32 NGDL, P 0.001. Both means fell within the normal reference range. This is the thyroid version of the ferritin story published yesterday. The normal label on a blood result does not mean the level is optimal for hair follicle function. How does thyroid dysfunction cause hair loss? Thyroid hormones, primarily T3, triodothyrinine, and T4, thyroxine, directly regulate the hair growth cycle through several mechanisms. T4 prolongs the antigen phase, keeping hair in active growth longer. When thyroid hormones drop in hypothyroidism, the antigen phase shortens, meaning hair grows for a reduced period before entering rest. T3 stimulates the division of keratinocyte stem cells in the hair bulb, insufficient T3 slows this proliferation, producing thinner, weaker hair shafts. Adequate thyroid hormone levels are also necessary to signal resting follicles to re-enter the growth phase. Without this signal, follicles remain dormant for extended periods. The result is diffuse hair thinning across the entire scalp, not pattern loss, alongside other hypothyroid symptoms, including fatigue, cold intolerance, dry skin, constipation, and weight gain. Hair affected by hypothyroidism tends to be dry and coarser than normal, often breaking more easily. In hyperthyroidism, the hair tends to be fine and fragile, with diffuse shedding driven by accelerated cycling. What is the Hertog sign? And why does it matter? Loss of the outer third of the eyebrows, known as Herthog's sign, combined with diffuse scalp thinning is a hallmark indic, a tour that thyroid dysfunction, not genetics, is driving hair fall. This is one of the most clinically useful physical signs in hair loss evaluation, because it is visible, requires no blood test, and is specific enough to immediately focus the investigation. Aga does not cause outer eyebrow thinning. Alopecia ariata can, but produces patches rather than the progressive outer third pattern. Herthog's sign, named for the Belgian dermatologist who described it, is the physical sign that should prompt thyroid evaluation before any blood work is ordered. If you look in the mirror and the outer third of your eyebrows is visibly thinner than it was a few years ago, that is a clinical signal, not just a grooming observation. It belongs in the conversation with your doctor. Why is TSH-only testing not enough? TSH, thyroid stimulating hormone, is the pituitary signal to the thyroid. It rises when thyroid hormone is low, telling the thyroid to produce more, and falls when thyroid hormone is high. It is useful as a screening test, but it has significant limitations as the only thyroid test ordered. The most important limitation for hair loss specifically. TSH often changes before actual thyroid hormone levels T3 and T4 drop, acting as an early warning system for dysfunction, but a normal TSH with suboptimal free T3 can exist when T4 to T3 conversion is impaired. T4 is the inactive form of thyroid hormone. It must be converted to T3, the active form, primarily in the liver and D kidneys. Normalizing thyroid function through comprehensive testing of TSH, free T3, and free T4, plus using combination T4-T3 therapy instead of T4 only, stops hair loss in thyroid patients. T4 only treatment leaves some patients with low T3 despite normal TSH. A patient on levothyroxine, T4 replacement, with a normal TSH may have suboptimal free T3 because their conversion is impaired, and their hair loss may be driven by that T3 deficit despite the technically treated thyroid. This is why the full panel matters TSH, free T3, Free T4, and thyroid antibodies, TPO and thyroglobulin, to detect autoimmune thyroid disease. 50% of patients with untreated thyroid disorders experience hair loss, making thyroid the most common endocrine cause of diffuse hair shedding, British Journal of Dermatology, 2008. Both normal TSH and Free T4 were within normal range in women with hair loss in the 2025 diseases study, yet both were significantly lower than healthy controls. P0.001. Biotin interferes with the lab immunoassay and can produce falsely low TSH, mimicking hyperthyroidism. Does biotin interfere with thyroid tests? Yes, and this is directly relevant to anyone taking hair, skin, and nail supplements. Biotin is found in almost all hair, skin, and nails vitamins. It can chemically interfere with lab equipment, causing a falsely low TSH reading that mimics hyperthyroidism, medication interference, biotin supplements.