Thyroid Problems and Hair Loss: Understanding the Connection

How does thyroid disease affect hair?

The thyroid gland produces hormones that regulate metabolism throughout the body, and hair follicles are highly sensitive to thyroid hormone levels. When thyroid hormone is too low or too high, follicles are pushed out of their active growing phase (anagen) prematurely into the resting and shedding phase (telogen). This produces diffuse thinning across the whole scalp rather than distinct patches — a pattern called telogen effluvium 1Ref 1Jonklaas J, Bianco AC, Bauer AJ, et al. (2014).Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement.Hypothyroidism mechanism affecting hair follicles via telogen effluvium, Hashimoto's as autoimmune hypothyroidism, standard TSH/T4 testing, and levothyroxine treatment and recovery timeline4Ref 4Rebora A (2019).Telogen effluvium: a comprehensive review.Telogen effluvium as the mechanism by which thyroid disease causes diffuse hair loss; delay of two to four months between trigger and visible shedding.

Because hair growth cycles run on a delay of two to four months, hair loss from a thyroid problem may show up weeks to months after levels first went out of range. Similarly, hair may not fully recover until months after thyroid levels are stabilized with treatment.

How does hypothyroidism (underactive thyroid) affect hair?

Hypothyroidism is the more commonly associated thyroid cause of hair loss. It occurs when the thyroid produces too little hormone. Hair may become dry, coarse, and brittle in addition to shedding diffusely. Other accompanying symptoms can include persistent fatigue, weight gain, feeling cold even in warm environments, constipation, depression, and dry skin.

A thinning or loss of the outer third of the eyebrows is sometimes noted with hypothyroidism, though this sign is not universal. Hashimoto's thyroiditis — an autoimmune form of hypothyroidism — is among the most common thyroid conditions and is diagnosed with TSH levels combined with thyroid antibody testing 1Ref 1Jonklaas J, Bianco AC, Bauer AJ, et al. (2014).Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement.Hypothyroidism mechanism affecting hair follicles via telogen effluvium, Hashimoto's as autoimmune hypothyroidism, standard TSH/T4 testing, and levothyroxine treatment and recovery timeline.

With appropriate treatment — typically levothyroxine to restore normal hormone levels — most people see hair shedding slow within a few months and gradual regrowth over six months to a year 1Ref 1Jonklaas J, Bianco AC, Bauer AJ, et al. (2014).Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement.Hypothyroidism mechanism affecting hair follicles via telogen effluvium, Hashimoto's as autoimmune hypothyroidism, standard TSH/T4 testing, and levothyroxine treatment and recovery timeline.

How does hyperthyroidism (overactive thyroid) affect hair?

An overactive thyroid produces too much hormone, speeding up body processes. Hair loss in hyperthyroidism tends to be fine and diffuse. Other symptoms can include unexplained weight loss despite normal or increased appetite, heart palpitations, feeling hot or sweating more than usual, anxiety, tremor, and irregular periods.

Graves' disease is the most common autoimmune cause of hyperthyroidism. Both the disease itself and some antithyroid medications used to treat it can affect hair. A clinician can help sort out what is driving the change and adjust treatment accordingly.

What should you do first: see primary care or a dermatologist?

A simple blood test — measuring TSH (thyroid-stimulating hormone), and sometimes free T4 and T3 — is the standard first step for evaluating thyroid function and is routinely ordered by primary care clinicians without a specialist referral 1Ref 1Jonklaas J, Bianco AC, Bauer AJ, et al. (2014).Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement.Hypothyroidism mechanism affecting hair follicles via telogen effluvium, Hashimoto's as autoimmune hypothyroidism, standard TSH/T4 testing, and levothyroxine treatment and recovery timeline.

Iron deficiency is another common and treatable contributor to diffuse hair loss that is worth checking at the same time, particularly in women with heavy periods or dietary restrictions 2Ref 2Leung AKC, Lam JM, Wong AHC, Hon KL, Li X (2024).Iron Deficiency Anemia: An Updated Review.Iron deficiency as a common and treatable co-contributor to diffuse hair loss, worth checking alongside thyroid function in women. A complete blood count and ferritin level are often ordered alongside the thyroid panel. Low ferritin can cause or worsen hair loss even in the absence of frank anemia, and ferritin levels in the lower half of the normal range are sometimes associated with hair shedding.

If thyroid levels and iron come back normal, and hair loss continues, a dermatologist can look further — including for androgenetic alopecia, which is very common and can coexist with thyroid disease 3Ref 3American Academy of Dermatology (2024).Hair Loss Resource Center.Recommendation to seek dermatology evaluation when hair loss continues after thyroid treatment; potential co-existence of androgenetic alopecia with thyroid disease. If thyroid disease is confirmed and treated, but hair loss persists more than a year after levels have been stable, dermatology evaluation is the appropriate next step.

Can thyroid medication itself cause hair loss?

Levothyroxine can occasionally cause temporary shedding early in treatment as the body adjusts to restored hormone levels — this is usually brief and resolves. More commonly, if hair loss continues or worsens on levothyroxine, it signals that the dose may need adjustment or that another factor is contributing 1Ref 1Jonklaas J, Bianco AC, Bauer AJ, et al. (2014).Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement.Hypothyroidism mechanism affecting hair follicles via telogen effluvium, Hashimoto's as autoimmune hypothyroidism, standard TSH/T4 testing, and levothyroxine treatment and recovery timeline.

Antithyroid medications used in hyperthyroidism (such as methimazole or propylthiouracil) can also occasionally contribute to hair changes. Work with your prescribing clinician to evaluate the cause rather than stopping medication without guidance.