Do finasteride (Propecia) or minoxidil (Rogaine) cause elevated thyroid test levels, specifically hyperthyroidism?

Finasteride and Minoxidil Effects on Thyroid Test Levels

Neither finasteride nor minoxidil is known to cause elevated thyroid test levels or hyperthyroidism based on available evidence. The FDA drug label for finasteride specifically indicates it has no effect on thyroid hormone levels 1.

Finasteride and Thyroid Function

Finasteride is a 5α-reductase inhibitor used to treat androgenetic alopecia (male pattern baldness) and benign prostatic hyperplasia. According to the FDA drug label:

• Finasteride "has no effect on circulating levels of cortisol, prolactin, thyroid-stimulating hormone, or thyroxine" 1
• The medication primarily affects dihydrotestosterone (DHT) levels by inhibiting the 5α-reductase enzyme
• While finasteride can affect certain hormone levels (particularly DHT, testosterone, LH, and FSH), thyroid hormones are not among those affected 1

Minoxidil and Thyroid Function

Minoxidil is a vasodilator that promotes hair growth through multiple pathways:

• There is no evidence in the provided literature that minoxidil affects thyroid hormone levels
• Minoxidil acts through vasodilation, anti-inflammatory effects, Wnt/β-catenin signaling, and possibly mild anti-androgenic effects 2
• Side effects of minoxidil are primarily related to hypertrichosis (excessive hair growth) and cardiovascular effects rather than thyroid dysfunction 2

Drugs Known to Affect Thyroid Function

While finasteride and minoxidil are not associated with thyroid test abnormalities, it's worth noting that other medications can affect thyroid function:

• Amiodarone is a well-known medication that can cause both hyper- and hypothyroidism 3
• Tyrosine kinase inhibitors can affect thyroid function, either directly or by altering thyroid hormone metabolism 3
• Immune modulators used for various conditions can cause thyroid dysfunction through immune or non-immune mechanisms 3

Clinical Implications

When evaluating thyroid function in patients taking finasteride or minoxidil:

• Abnormal thyroid test results should prompt investigation for other causes rather than attributing them to these medications
• Common causes of hyperthyroidism include Graves' disease, Hashimoto's thyroiditis, and functional thyroid nodules 4
• Risk factors for hyperthyroidism include female sex, advancing age, black race, low iodine intake, personal or family history of thyroid disease 4

Monitoring Recommendations

For patients on finasteride or minoxidil:

• Routine monitoring of thyroid function is not necessary based solely on the use of these medications
• If thyroid dysfunction is suspected based on clinical symptoms, appropriate testing should include serum TSH as the primary screening test 4
• Multiple tests should be done over a 3-6 month interval to confirm abnormal findings 4

It's important to note that while these medications don't affect thyroid function, they may have other hormonal effects. Finasteride can alter testosterone levels and sexual function, while minoxidil's side effects are primarily cardiovascular and cosmetic in nature 2, 5.