Effect of Finasteride on Serum Level of Thyroxine in a Woman Treated for Pattern Hair Loss

ABSTRACT

Finasteride, traditionally used for male pattern hair loss, is also used to treat hirsutism, acne, and female pattern hair loss (FPHL) in women. In men, adverse effects include decreased libido, erectile dysfunction, testicular tenderness, pollakiuria, gynecomastia, and depression, with some cases of persistent sexual issues even after discontinuation (postfinasteride syndrome) and suicidal ideation. For women, the primary concern is teratogenicity in those of childbearing age. We present a case of a postmenopausal woman prescribed oral finasteride for FPHL who needed to increase her levothyroxine dosage to maintain normal thyroid hormone levels during treatment. Thyroid diseases, more common in women from puberty to menopause, are influenced by estrogen, which increases thyroxine-binding globulin (TBG) and alters thyroid hormone needs in hypothyroid women. Although finasteride does not directly affect estrogen, it lowers dihydrotestosterone, potentially increasing circulating testosterone. This excess testosterone can convert to estrogen, further skewing hormonal balance in older women. Elevated estrogen levels increase TBG concentration, reducing free thyroid hormones and causing hypothyroid symptoms. Considering these interactions, especially in postmenopausal women with hypothyroidism and hair loss, careful assessment is crucial before prescribing oral finasteride.