Thyroid Disease and Hair Loss: What Is the Connection?

How does thyroid disease cause hair loss?

Hair follicles cycle through growth (anagen), transition (catagen), and resting (telogen) phases. Thyroid hormones — T3 and T4 — regulate the activity of hair follicles. When thyroid levels are significantly out of range, more hairs than usual enter the resting phase simultaneously, leading to diffuse shedding two to three months later. This pattern is called telogen effluvium 1Ref 1Rebora A (2019).Telogen effluvium: a comprehensive review.Telogen effluvium mechanism, thyroid dysfunction as a trigger, and expected timeline for hair regrowth.

The hair loss associated with thyroid disease is typically diffuse — spread evenly across the scalp rather than in patches — and affects the full thickness of hair rather than just the hairline. You might notice more hair in the shower, on your pillow, or when brushing.

Hypothyroidism and hair loss

Hypothyroidism (underactive thyroid) is one of the recognized triggers of telogen effluvium. In addition to diffuse scalp thinning, people with hypothyroidism may also notice:

• Loss of the outer third of the eyebrows (a classic but not universal finding)
• Dry, coarse, or brittle hair texture
• Slow-growing hair

Hypothyroidism is commonly caused by Hashimoto's thyroiditis, an autoimmune condition in which the immune system attacks the thyroid gland 2Ref 2Weetman AP (2021).An update on the pathogenesis of Hashimoto's thyroiditis.Hashimoto's thyroiditis as the most common autoimmune cause of hypothyroidism with hair loss3Ref 3Jonklaas 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.Thyroid testing as part of evaluation for unexplained hair loss with other hypothyroid symptoms; treatment leading to hair recovery. Because autoimmune conditions sometimes overlap, it is worth knowing that alopecia areata — patchy hair loss — has its own autoimmune basis distinct from thyroid disease, though the two conditions can occasionally co-occur.

Hyperthyroidism and hair loss

Hyperthyroidism (overactive thyroid) can also cause hair thinning, often with a fine texture and faster hair cycling. Graves' disease — the most common cause of hyperthyroidism — is an autoimmune condition and can coexist with other autoimmune hair disorders 4Ref 4Ross DS, Burch HB, Cooper DS, Greenlee MC, Laurberg P, Maia AL, Rivkees SA, Samuels M, Sosa JA, Stan MN, Walter MA (2016).2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis.Graves' disease as autoimmune cause of hyperthyroidism associated with hair loss. Hair loss in hyperthyroidism often improves once thyroid levels are brought into the normal range with treatment.

Is thyroid testing worth it for hair loss?

Hair loss has many causes, and thyroid disease is one of them — but far from the only one. Other common causes of diffuse hair shedding include:

• Iron deficiency anemia 5Ref 5Leung AKC, Lam JM, Wong AHC, Hon KL, Li X (2024).Iron Deficiency Anemia: An Updated Review.Iron deficiency anemia as a common alternative cause of hair shedding in the differential diagnosis
• Nutritional deficiencies (low vitamin D, low zinc, low protein intake)
• Significant physical or emotional stress
• Postpartum shedding (common in the months after delivery)
• Certain medications
• Other hormonal changes (menopause, stopping hormonal birth control)
• Androgenetic alopecia (hereditary pattern hair thinning)

A thyroid test (TSH) is a reasonable part of the evaluation for unexplained diffuse hair loss — especially with other symptoms like fatigue, weight changes, or cold intolerance — but it should be part of a broader workup, not the only test 3Ref 3Jonklaas 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.Thyroid testing as part of evaluation for unexplained hair loss with other hypothyroid symptoms; treatment leading to hair recovery.

Will treating thyroid disease reverse hair loss?

In most cases, yes — but patience is required. Hair follicles that entered the resting phase will not immediately re-enter the growth phase when thyroid levels normalize. Hair regrowth typically begins within three to six months of treating the underlying thyroid condition and may take up to a year to return to baseline density.

If hair loss continues after thyroid function has been stable and normal for six or more months, other causes should be explored — whether that is nutritional, hormonal, or a separate hair condition like androgenetic alopecia or alopecia areata 1Ref 1Rebora A (2019).Telogen effluvium: a comprehensive review.Telogen effluvium mechanism, thyroid dysfunction as a trigger, and expected timeline for hair regrowth3Ref 3Jonklaas 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.Thyroid testing as part of evaluation for unexplained hair loss with other hypothyroid symptoms; treatment leading to hair recovery.

When to see a clinician

See a primary care clinician if: - Hair loss is significant, rapid, or distressing - You have other symptoms that could suggest thyroid disease (fatigue, temperature sensitivity, weight changes, heart rate changes) - Hair loss occurs in patches rather than diffusely (this pattern is more typical of alopecia areata) - You are postpartum and losing significant hair beyond the expected few months

A Gale primary care clinician can order thyroid labs, iron studies, and other relevant testing, interpret results in context, and refer you to a dermatologist if the evaluation points in that direction. Dermatologists specialize in hair loss diagnosis and can perform scalp biopsies when the cause is unclear.