Hypothyroidism occurs when the thyroid gland does not produce enough hormones, resulting in a slowed metabolism. Many people with this condition wonder if it directly causes hirsutism, which is the unwanted growth of dark, coarse facial or body hair in a male-like pattern. The link between hypothyroidism and hirsutism is not a direct cause-and-effect relationship, but an indirect one involving the regulation of other hormones.
The Link Between Thyroid Function and Hair Growth
Thyroid hormones (T4 and T3) regulate the body’s metabolism and general health, including the health of hair follicles. They are not the direct drivers of hair growth in androgen-dependent areas. The thyroid’s influence on unwanted hair is subtle and occurs through its effect on liver function and binding proteins.
Hypothyroidism affects the liver’s production of Sex Hormone-Binding Globulin (SHBG). SHBG is a protein that binds to sex hormones, including testosterone, making them inactive. When the thyroid is underactive, the liver’s synthesis of SHBG decreases significantly.
When less SHBG is available, more “free” or biologically active testosterone circulates. This increase in free testosterone amplifies hyperandrogenism, which is the true trigger for hirsutism. While hypothyroidism rarely causes hirsutism alone, it can worsen the condition for those already predisposed to it.
The True Hormonal Drivers of Unwanted Hair Growth
Hirsutism is driven by high levels of androgens, which are male sex hormones like testosterone. Androgens stimulate fine vellus hairs to transform into thick, dark terminal hairs characteristic of male-pattern growth, such as on the chin, upper lip, chest, and back. Hirsutism is a common condition affecting up to 10% of women, and it typically signals an underlying endocrine imbalance.
The vast majority of hirsutism cases (75% to 90%) are caused by Polycystic Ovary Syndrome (PCOS). PCOS is a complex endocrine disorder where the ovaries produce excess androgens, often combined with irregular menstrual cycles, acne, and insulin resistance. Medical professionals use the Ferriman-Gallwey score to grade the severity of hair growth when diagnosing the condition.
Other, less common causes of androgen excess also drive unwanted hair growth. These include non-classic congenital adrenal hyperplasia (NCAH), a genetic condition affecting adrenal hormone production, or, rarely, an androgen-secreting tumor. When excessive hair growth occurs without an identifiable hormonal imbalance, it is classified as idiopathic hirsutism, suggesting greater hair follicle sensitivity to normal androgen levels.
Identifying and Treating Underlying Imbalances
Anyone experiencing a noticeable increase in facial or body hair should consult a healthcare provider, such as an endocrinologist or gynecologist, for a thorough diagnostic workup. Identifying the cause relies on blood tests measuring key hormone levels, including TSH and free T4 for thyroid function, and total and free testosterone to evaluate for hyperandrogenism.
Treatment addresses both the underlying hormonal condition and the hair growth itself. If hypothyroidism is diagnosed, daily hormone replacement medication restores TSH and T4 levels to the normal range, which helps normalize SHBG levels. For hirsutism caused by PCOS, management involves lifestyle changes, weight loss, or medications that improve insulin sensitivity.
To manage hair growth directly, anti-androgen medications, such as spironolactone, may be prescribed to block the effects of male hormones on the hair follicle. Oral contraceptives are also commonly used, as they help regulate the menstrual cycle and increase SHBG, reducing free testosterone. Existing hair often requires cosmetic removal methods like laser therapy or electrolysis for long-term reduction.