Thyroid disease causes hair loss, and the pattern is specific enough to be useful. Both hypothyroidism and hyperthyroidism produce diffuse shedding across the whole scalp rather than defined bald patches. The National Institute of Diabetes and Digestive and Kidney Diseases lists dry, thinning hair among the symptoms of hypothyroidism. A 2023 Cureus review of thyroid dysfunction and hair disorders describes hyperthyroidism, hypothyroidism and drug-induced hypothyroidism as capable of inducing widespread hair shedding.

Why thyroid hair loss is diffuse

Hair grows in cycles, and thyroid hormone influences how long follicles stay in the growing phase. When thyroid hormone is too low or too high, a larger than usual proportion of follicles shift into the resting phase at the same time and then shed together. That is telogen effluvium, and it produces thinning across the entire scalp with more hair in the drain and on the pillow. It does not produce round smooth patches, which point instead to alopecia areata.

The autoimmune overlap

Alopecia areata is itself autoimmune and clusters with autoimmune thyroid disease, which is why patchy hair loss in someone with Hashimoto's is worth showing to a dermatologist rather than assuming it is a dosing issue. Female pattern hair loss, which thins the crown and widens the part while sparing the frontal hairline, is a separate condition that thyroid treatment will not fix.

What else to check before blaming the thyroid

Iron deficiency is the most common competing cause and can exist with a normal haemoglobin, so ferritin needs to be measured directly. Other frequent triggers are a febrile illness or surgery two to three months earlier, rapid weight loss, delivery of a baby, starting or stopping hormonal contraception, and low protein intake. Each of these produces the same diffuse shedding with a delay of roughly two to three months from the trigger.

Pattern	Suggests	Next step
Diffuse shedding across the whole scalp	Telogen effluvium: thyroid, iron, illness, postpartum	TSH, free T4, ferritin
Round smooth bald patches	Alopecia areata	Dermatology referral
Widening part with crown thinning	Female pattern hair loss	Dermatology, androgen assessment if other signs
Coarse dry hair with cold intolerance and constipation	Hypothyroidism	TSH and free T4
Fine hair with palpitations and heat intolerance	Hyperthyroidism	TSH, free T4, total T3

Ask for TSH, free T4 and ferritin on one draw, and note what happened two to three months before the shedding started, because that is usually where the trigger sits. Stop biotin at least 48 hours before the blood test, since it distorts thyroid assays and does not treat thyroid-related shedding.

Telogen effluvium follows its trigger by roughly two to three months and is usually self-limiting once the trigger is corrected. A positive gentle hair pull test with predominantly telogen club hairs supports the diagnosis. Ferritin should be measured directly since iron deficiency frequently coexists; many dermatology sources treat ferritin below 30 ng/mL as deficient and target higher values in active shedding, though the threshold for hair-specific benefit is not established by randomised trial.

Correcting thyroid function is the treatment for thyroid-related shedding, and shedding can continue for some months after biochemical correction because the hair cycle lags. Over-replacement is itself a cause: a suppressed TSH on levothyroxine produces the same diffuse shedding as untreated hyperthyroidism. Check for coexisting coeliac disease, B12 deficiency, and androgen excess where clinical features suggest it.

Sources

Why thyroid hair loss is diffuse

The autoimmune overlap

What else to check before blaming the thyroid

Sources

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