Introduction Hypogonadism is a disease to cause decreased energy, libido loss, body hair loss, mood depression, muscle volume reduction, and hot flashes [1-3]. 5α-reductase inhibitors (5-ARIs) are administered orally for male pattern alopecia and benign prostatic hyperplasia due to decrease of dihydrotestosterone, a potent male hormone [4]. 5-ARIs are generally known not to cause hypogonadism because 5-ARIs do not decrease total testosterone (TT) [4-8]; decreased testosterone causes hypogonadism. In recent years, 2 reports suggested that 5-ARIs may decrease TT to cause symptoms of hypogonadism [9,10], but there is no report that 5-ARIs caused cold sensation and paresthesia of lower extremities. We here report a patient with strong cold sensation and paresthesia of lower extremities and symptoms of hypogonadism due to 2 types of oral 5-ARIs. The one was finasteride, an imported medicine for personal use, and the other was saw palmetto, an over-the-counter drug in Japan. Case Presentation A 28-year-old previously healthy man visited our hospital with strong cold sensation and paresthesia of lower extremities for 3 months, and additional symptoms included palpitations, headache, and fatigue. He had been taking 2 tablet-types of 5-ARIs (finasteride (1 mg/day) and saw palmetto (fruit extract 320 mg/day)) for alopecia prevention in the 4 months before his visit. On physical examination, we found no abnormalities in his thyroid gland and limbs. Laboratory testing revealed TT, follicle-stimulating hormone (FSH), and luteinizing hormone (LH) levels of 160 ng/dL, 2.4 mIU/mL, 8.4 mIU/mL, respectively. TT was under the lower end of the range for his age. We diagnosed him with hypogonadism due to a lack of testosterone secretion. Eight days after he stopped taking both drugs, TT, FSH and LH levels improved to 404 ng/dL, 2.3 mIU/mL and 5.1 mIU/mL, respectively. After a month, all symptoms were completely resolved. Discussion From our patient, we have learned the following: 5-ARIs can cause symptoms of hypogonadism. 5-ARIs can cause cold sensation and paresthesia of lower extremities. Medical professionals should review every drug as a potential source of symptoms. Firstly, 5-ARIs can cause symptoms of hypogonadism. In general, primary hypogonadism that develops after puberty is diagnosed by both decreases in TT value and its consistent symptoms [2]; however, it may be under diagnosed due to 2 reasons. First, signs and symptoms of hypogonadism are often unspecific [1]. Second, TT value should be evaluated considering Volume 1 Issue 3-2017 Abstract Hypogonadism causes libido loss, body hair loss, mood depression, muscle volume reduction, and hot flashes. These symptoms can be due to lack of testosterone secretion from the testes or of testosterone action in target organs. 5α-reductase inhibitors (5-ARIs), often contained in hair growth-promoting agent, are known not to cause hypogonadism because they reduce dihydrotestosterone and do not decrease total testosterone (TT). We here report a patient with strong cold sensation and paresthesia of lower extremities and hypogonadism caused by 5-ARIs taken orally. A 28-year-old man visited our hospital with fatigue and strong cold sensation and paresthesia of lower extremities. He was taking 2 tablet-types of 5-ARIs. The one was finasteride (1 mg/day), an imported medicine for personal use, and the other was saw palmetto (fruit extract 320 mg/ day), an over-the-counter drug in Japan. Laboratory testing revealed TT, follicle-stimulating hormone (FSH), and luteinizing hormone (LH) levels of 160 ng/dL, 2.4 mIU/mL and 8.4 mIU/mL, respectively. We diagnosed hypogonadism due to a lack of testosterone secretion. Eight days after discontinuing both drugs, TT, FSH and LH levels improved to 404 ng/dL, 2.3 mIU/mL and 5.1 mIU/mL, and his symptoms improved completely in a month. Physicians should consider 5-ARIs as a cause of cold sensation and paresthesia of lower extremities and carefully check every ingested drug including imported and over-the-counter drugs.