A systematic review published in Cureus in August 2023 and archived on NIH's PubMed Central concludes that both an underactive and an overactive thyroid can trigger diffuse hair shedding and change the way hair looks and feels.
The authors add that shedding can start months before other thyroid symptoms show up, which is why the scalp is often the first place a hormonal shift becomes visible.
The pattern of change is not the same for both conditions.
Hypothyroidism, meaning the thyroid produces too little hormone, tends to make hair slow-growing, coarse, dry and brittle, and can thin the outer third of the eyebrow.
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Hyperthyroidism, meaning the thyroid produces too much hormone, tends to leave hair fine and silky but weaker at the shaft.
Diffuse, non-scarring shedding across the whole scalp is possible in either direction.
Why the thyroid shows up in your hairbrush
Thyroid hormones help regulate the growth phase of hair follicles.
When levels drift too low or too high, more follicles switch out of active growth and into the shedding phase at the same time.
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That produces the pattern dermatologists call telogen effluvium, meaning a prolonged period of increased shedding rather than bald patches.
The US National Institute of Diabetes and Digestive and Kidney Diseases lists dry, thinning hair among the recognized symptoms of hypothyroidism, alongside fatigue, cold intolerance and weight gain.
The agency also warns that the condition develops slowly, so symptoms can go unnoticed for months or years, and that diagnosis cannot rest on symptoms alone.
Nutrient gaps that mimic a thyroid problem
Hair loss driven by low iron, low vitamin D or low zinc can look identical to the shedding caused by a thyroid problem, which is why doctors usually order both panels together.
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A study of adults with diffuse hair loss, published in Advances in Dermatology and Allergology and indexed on PubMed Central, found that 20.4 percent of patients had low ferritin and 79.6 percent had low vitamin D.
The authors concluded that both markers should be checked and corrected before starting any hair loss treatment.
Ferritin is the protein that stores iron.
Standard labs flag deficiency below roughly 15 ng/mL, but studies in women with telogen effluvium report shedding at levels well above that cutoff, which is why dermatologists often use a higher target when hair is the presenting complaint.
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Vitamin D and zinc have both been linked to hair follicle cycling in a broader NIH review of diet and hair loss, though the review notes that data on supplementing these nutrients specifically for hair loss remains limited.
What to ask your doctor for
If your shedding is sudden, persistent for more than a few weeks, or paired with fatigue, weight change, feeling cold or feeling wired, ask your doctor about a thyroid panel that includes TSH, and a basic nutrient workup that covers ferritin and vitamin D.
Zinc can be added if diet or absorption is a concern.
These are ordinary blood tests, and the results tell your doctor whether the hair change is a thyroid signal, a nutrient gap or something else that needs a closer look.
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Welltica has previously covered other warning signs that a thyroid may be off, and hair change belongs on that list.
Do not start iron, zinc or thyroid medication on your own.
Iron overload has its own risks, and taking thyroid hormone without a confirmed diagnosis can push a healthy gland out of balance.
The test is the first step, not the supplement aisle.
This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.