© 2016 AWHONN Hypothyroidism, a disease in which the thyroid gland does not make enough thyroid hormone, is the second most common endocrine disorder among women. Symptoms of hypothyroidism include fatigue, weight gain, alteration in cognition, infertility, and menstrual abnormalities. The most common cause of hypothyroidism in the United States is Hashimoto's thyroiditis. The American Thyroid Association recommends an initial screening for thyroid disease at age 35 years and every 5 years thereafter. Thyroid-stimulating hormone is highly sensitive to thyroid dysfunction and is used to evaluate thyroid disorders. Monotherapy with levothyroxine is the standard for treating hypothyroidism. Diagnosing hypothyroidism requires appropriate diagnostic tests to facilitate prompt diagnosis and treatment.