Brittle Hair and Thyroid Dysfunction
Brittle hair is associated with hypothyroidism (underactive thyroid), not hyperthyroidism. 1
Key Clinical Features of Hypothyroidism and Hair
Hair loss is a well-established symptom of hypothyroidism, affecting a significant proportion of patients with this condition 1. The American Academy of Family Physicians recognizes hair loss as a common manifestation of decreased thyroid hormone activity 1.
Specific Hair Changes in Hypothyroidism
- Coarse scalp hair develops in approximately 29% of hypothyroid patients, representing a change in hair texture that makes hair brittle and difficult to manage 2
- Diffuse hair loss occurs in 46% of hypothyroid patients, often presenting as thinning across the entire scalp rather than in specific patterns 2
- Dull and sparse hair appearance results from disordered anagen (growth) phase of the hair cycle 3
- Dry, coarse skin accompanies the hair changes in approximately 65% of hypothyroid patients, contributing to the overall brittle appearance 2
Mechanism Behind Hair Changes
The thyroid hormones (thyroxine and triiodothyronine) control cellular and tissue metabolism throughout the body 3. When thyroid hormone levels are low, hair follicle cell proliferation decreases, leading to impaired hair growth and cycling 4. This metabolic slowdown affects the anagen phase of hair growth, resulting in brittle, coarse hair that breaks easily 3.
Important Clinical Distinction
Hyperthyroidism causes different hair manifestations - primarily disordered growth of hair and nails due to increased metabolic turnover, but not the characteristic brittle, coarse texture seen in hypothyroidism 3. Hyperthyroid patients may experience hair changes, but these are related to accelerated metabolism rather than the structural brittleness characteristic of hypothyroidism 5.
Clinical Pitfall
Hair loss attributed to hypothyroidism may not improve with thyroxine replacement alone if concurrent zinc deficiency exists 6. Zinc is required for thyroid hormone synthesis, and thyroid hormones are essential for zinc absorption, creating a potential vicious cycle 6. Consider zinc supplementation if hair loss persists despite adequate thyroid hormone replacement 6.