Gynaecomastia Appeared Three Days after Starting Methimazole

We reported that in a 29-year-old male patient with hyperthyroidism, bilateral breast swelling appeared in three days after starting methimazole and gradually aggravated. Several days later, a small amount of transparent liquid could be squeezed out from bilateral mammary glands. Breast ultrasound confirmed gynaecomastia. The level of testosterone, estradiol and luteinizing hormone increased. After the patient continued taking methimazole for a while, gynaecomastia relieved. Testosterone, luteinizing hormone and thyroid functions restored to normal. The possible mechanisms included increased levels of serum total cholesterol and relatively decreased T3 after initiating methimazole.