Hypothyroidism is a systemic metabolic disorder resulting from inadequate production of thyroid hormone (TH) or inadequate action of TH to target tissues. Primary hypothyroidism, as the principal type, is mainly caused by Hashimoto￠s thyroiditis, while, other causes include central deficiency of thyrotropin-releasing hormone or thyroid-stimulating hormone (TSH), or the outcome after the thyroid surgery or radioiodine therapy for hyperthyroidism. It is difficult to diagnose thyroid diseases based only on symptoms, but manifestation of these symptoms could be an indicator for thyroid function evaluation. Evaluations of TSH and free thyroxine (FT4) is used to diagnose overt hypothyroidism (high TSH with low FT4 levels) and subclinical hypothyroidism (high TSH with normal FT4 levels). The goal to treat hypothyroidism is to relieve symptoms and avoid progression to myxedema. Hypothyroidism is usually treated with synthetic levothyroxine. Although the diagnosis and treatment of hypothyroidism is often considered simple, there are large numbers of people with hypothyroidism who are suboptimally treated. Even in those people with biochemically euthyroid on levothyroxine replacement, there is a significant proportion who report poorer quality of life. This review summarized etiology and types of hypothyroidism, including subclinical disease, and diagnosis and current treatment for hypothyroidism. The reasons for suboptimal treatment and potential solutions were also discussed.