Can hypothyroidism or hyperthyroidism cause diffuse hair loss in an adult?

Both Hypothyroidism and Hyperthyroidism Cause Diffuse Hair Loss

Both hypothyroidism and hyperthyroidism can cause diffuse hair loss in adults, with hypothyroidism being the more commonly recognized cause. 1

Hypothyroidism and Hair Loss

Mechanism and Clinical Presentation

• Hair loss is a cardinal symptom of hypothyroidism, occurring due to decreased metabolic rate and reduced thyroid hormone activity affecting hair follicle function 1
• Thyroid hormones (T3 and T4) directly regulate human hair follicle biology, controlling hair matrix keratinocyte proliferation, anagen duration, and hair pigmentation 2
• T4 specifically prolongs the anagen (growth) phase by down-regulating TGF-beta2, a key anagen-inhibitory growth factor 2
• When thyroid hormone levels are insufficient, hair follicles prematurely enter telogen (resting phase), resulting in diffuse shedding 2

Clinical Evidence

• Hypothyroidism is a well-recognized and common cause of diffuse hair loss, with hair loss being one of the classic presenting symptoms alongside fatigue, weight gain, cold intolerance, and constipation 1
• The hair loss in hypothyroidism may be severe and extensive, particularly when thyroid dysfunction is prolonged or untreated 3
• Hair loss attributed to hypothyroidism may not improve with levothyroxine alone unless zinc deficiency is also addressed, as thyroid hormones are essential for zinc absorption 3

Associated Conditions

• Autoimmune thyroid disease (Hashimoto's thyroiditis) associates with various types of alopecia, including alopecia areata, telogen effluvium, and potentially female pattern hair loss 4
• The prevalence of thyroid dysfunction in patients with telogen effluvium ranges from 5.7% to 17% 4

Hyperthyroidism and Hair Loss

Mechanism

• Excess thyroid hormone levels also result in altered hair structure and function, leading to effluvium (hair shedding) 2
• Hyperthyroidism accelerates the hair growth cycle, causing premature transition from anagen to telogen phase 5
• Drug-induced hyperthyroidism can similarly induce widespread hair shedding 5

Clinical Presentation

• Hair loss in hyperthyroidism typically presents as diffuse thinning rather than patchy loss 5
• The hair may also become finer in texture and more fragile 5

Diagnostic Approach

Initial Testing

• Measure TSH and free T4 to distinguish between hypothyroidism (elevated TSH, low free T4) and hyperthyroidism (suppressed TSH, elevated free T4) 6, 1
• TSH has sensitivity above 98% and specificity greater than 92% for detecting thyroid dysfunction 7

Additional Considerations

• Check for anti-thyroid peroxidase (TPO) antibodies to identify autoimmune etiology, which predicts higher risk of progression and may influence treatment decisions 7, 4
• Consider zinc levels in hypothyroid patients with hair loss, as zinc deficiency can compound the problem and prevent hair regrowth despite thyroid hormone replacement 3
• Evaluate for other autoimmune conditions in patients with autoimmune thyroid disease and alopecia areata 4

Treatment Implications

For Hypothyroidism

• Initiate levothyroxine therapy for TSH >10 mIU/L regardless of symptoms, as this level carries significant risk and will improve hair loss along with other symptoms 7
• For symptomatic patients with TSH 4.5-10 mIU/L, consider a trial of levothyroxine therapy with clear evaluation of benefit 7
• Hair regrowth typically occurs within 3-4 months of adequate thyroid hormone replacement, though zinc supplementation may be necessary for optimal results 3

For Hyperthyroidism

• Treatment of the underlying hyperthyroidism (whether from Graves' disease, toxic nodular goiter, or thyroiditis) typically resolves the hair loss 6
• In cases of thyroiditis-induced thyrotoxicosis, the condition is self-limiting and leads to hypothyroidism after approximately 1 month, requiring monitoring and potential levothyroxine therapy 6

Common Pitfalls

• Do not assume normal thyroid function based on a single test—30-60% of mildly abnormal TSH levels normalize on repeat testing, so confirm with repeat measurement after 3-6 weeks 7
• Avoid missing subclinical thyroid dysfunction (elevated or suppressed TSH with normal free T4), as even subclinical disease can cause hair loss 7, 4
• Never overlook concurrent zinc deficiency in hypothyroid patients with persistent hair loss despite adequate thyroid hormone replacement 3
• Consider that hair loss may be the presenting symptom of thyroid dysfunction before other classic symptoms become apparent 1