Hair loss with hypothyroidism

An underactive thyroid can cause increased shedding and thinning across the scalp. Once treatment restores thyroid hormone levels, thyroid-related shedding usually improves. Regrowth takes time, though, and persistent hair loss may have another cause alongside hypothyroidism.

How hypothyroidism affects your hair and scalp

In hypothyroidism, the body does not have enough thyroid hormone. This can disrupt the hair-growth cycle, with more follicles entering the resting, or telogen, phase. The result is generally diffuse shedding across the scalp rather than isolated bald patches. Review of thyroid dysfunction and hair disorders

Common hypothyroidism symptoms include fatigue, sensitivity to cold, weight gain, constipation, and dry skin. Hair loss alongside these symptoms can point toward a thyroid contribution. A change in hair texture may provide another clue. Review of hypothyroidism

What thyroid-related hair loss looks like

An underactive thyroid may increase shedding and change the texture of the hair that remains. Sometimes the eyebrows become thinner, too.

• More hair in the brush or shower drain. Shedding can continue as hairs leave the resting phase. Review of hair disorders
• Dry, coarse, brittle, or dull hair. Texture changes can accompany thinning. Review of hair disorders
• Thinning at the outer third of the eyebrows. This can be a clue, but it also occurs without thyroid disease. Review of hair disorders

A retrospective study examined records from 500 women with telogen effluvium, a form of diffuse shedding. Hair loss was more severe in those with hypothyroidism than in those with normal thyroid function. The finding supports a possible connection, but a retrospective comparison cannot establish why the difference occurred or prove that the thyroid caused each woman’s hair loss. Retrospective study

Why low thyroid hormone can cause shedding

Hair follicles respond to thyroid hormones. A laboratory study of human scalp follicles found that T4 prolonged the growth phase, called anagen, and increased the proliferation of hair-matrix cells. T3 and T4 also reduced programmed cell death. These findings came from follicles maintained in culture, not a clinical trial of hair regrowth. Laboratory study of human hair follicles

When a trigger disrupts the growth cycle, many follicles may enter the resting phase around the same time. The resulting shedding is called telogen effluvium, a common cause of diffuse hair loss. It typically becomes noticeable about 3–4 months after the trigger. That delay helps explain why hair may start looking thinner some time after thyroid hormone levels fall. Other triggers can produce the same pattern. Review of telogen effluvium

Thyroid-related hair loss versus other common causes

When the thyroid is responsible, thinning usually occurs across the scalp. Other conditions can also cause diffuse shedding. The distribution, timing, and accompanying symptoms help distinguish them.

Cause	Typical pattern	Other clues
Hypothyroidism	Diffuse thinning and increased shedding across the scalp	Dry, brittle hair; fatigue; cold sensitivity
Alopecia areata, an autoimmune condition	Small, round bald patches	Short, broken hairs at patch edges; eyebrows, eyelashes, or beard may also be affected
Iron deficiency	Diffuse thinning, studied mainly in women	Women with nonscarring hair loss have had lower ferritin levels on average in observational studies; this does not establish the cause in an individual
Female pattern hair loss	Thinning over the crown and part, often with a widening center part	Gradual progression, usually with the front hairline preserved
Shedding after stress or illness	Diffuse shedding several months after the event	Recent illness, surgery, childbirth, or a crash diet

Table sources: NIAMS on alopecia areata, systematic review and meta-analysis of iron deficiency and hair loss in women, MedlinePlus on female pattern hair loss, and review of telogen effluvium.

Hashimoto’s thyroiditis is an autoimmune thyroid disease and the most common cause of hypothyroidism in the United States. Thinning hair is one possible symptom. NIDDK on Hashimoto’s disease

Thyroid autoantibodies are more common in people with alopecia areata, and the two autoimmune conditions can coexist. Blood tests can help identify thyroid involvement, but they cannot distinguish every cause of hair loss on their own. Systematic review and meta-analysis

Which blood tests help investigate a thyroid cause?

The pattern of hair loss provides initial clues. Blood tests can establish whether thyroid dysfunction is present; deciding whether it explains the shedding also requires the rest of the clinical picture.

• Thyroid-stimulating hormone, or TSH: Usually the initial test. High TSH together with low free T4 indicates overt primary hypothyroidism. Review and TSH background.
• Free or total T4: T4 is a hormone released by the thyroid. Free T4 measures the unbound fraction; total T4 includes hormone attached to transport proteins. Clinicians generally interpret free T4 alongside TSH when evaluating primary hypothyroidism. Total T4 requires consideration of changes in binding proteins. Total T4 background.
• Thyroid peroxidase, or TPO, antibodies: These are present in most people with Hashimoto’s disease and help identify an autoimmune cause. NIDDK on Hashimoto’s disease.
• Ferritin and other iron studies: These help assess whether iron deficiency could be contributing. They do not rule out every nutritional cause of hair loss. Review of telogen effluvium and ferritin background.

Biotin supplements can interfere with some TSH and T4 assays. The Synthroid prescribing information advises stopping biotin-containing supplements at least two days before thyroid testing. Tell the clinician and laboratory what you take and follow their preparation instructions; the necessary interval may depend on the dose and assay. DailyMed prescribing information and review of thyroid test interference.

How hair regrows during hypothyroidism treatment

Thyroid-related shedding usually improves as treatment restores hormone levels, but hair regrows gradually. Levothyroxine is the usual first-line treatment for hypothyroidism. In adults with primary hypothyroidism, TSH is generally checked 6–8 weeks after starting treatment or changing the dose. Pregnancy and other clinical circumstances may require a different monitoring schedule. Review of hypothyroidism

Replacing the missing hormone can allow growth to recover, but normal laboratory values do not guarantee an immediate return to previous hair density. The cited hair-disorder review describes several overlapping causes of thinning; persistent loss should not automatically be attributed to permanently damaged follicles from an old thyroid problem. Review of thyroid dysfunction and hair disorders

Rarely, partial hair loss occurs during the first few months of levothyroxine treatment and is usually temporary. Discuss changes with the prescriber before changing or stopping the medication. If shedding continues after thyroid levels normalize, another cause may need investigation. DailyMed prescribing information

Why shedding can continue despite normal thyroid results

Hair may continue shedding for a while after thyroid levels normalize. A second cause may also be present. It matters which results are normal: a normal free T4 alone does not mean that all thyroid tests are normal.

• Subclinical hypothyroidism: TSH is elevated while free T4 is within range. This pattern has been reported in up to 10% of adults, although prevalence depends on age and the reference range used. It is not a fully normal thyroid panel, and the finding alone does not prove the cause of hair loss. Review, free T4 background, and subclinical hypothyroidism background.
• Positive TPO antibodies: These are associated with a greater risk of developing hypothyroidism later, but they do not by themselves explain current hair loss. Study of progression to hypothyroidism and TPO antibody background.
• Too much thyroid medication: Excess replacement can cause features of hyperthyroidism, including hair loss. This possibility is assessed using medication history and thyroid results; it is not an explanation to assume when the entire evaluation is normal. DailyMed prescribing information.
• Delayed shedding: Hairs that already entered the resting phase may fall out months after the original trigger. Review of telogen effluvium.
• Iron deficiency or female pattern hair loss: More than one condition can be present. Depending on the history and examination, assessing thyroid disease and iron deficiency can be part of the evaluation for thinning hair. MedlinePlus on female pattern hair loss.

Thyroid and iron tests help narrow the possibilities. The distribution of hair loss, its course over time, and a scalp examination remain just as important for understanding what is happening.

Sources

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• Bin Dayel S, Hussein RS, Atia T, Abahussein O, Al Yahya RS, Elsayed SH. Is thyroid dysfunction a common cause of telogen effluvium?: a retrospective study. Medicine (Baltimore). 2024;103(1):e36803. https://doi.org/10.1097/MD.0000000000036803 PubMed
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