Thyroid stimulating hormone (TSH) is a glycoprotein hormone produced by the anterior pituitary gland, which stimulates the thyroid gland to produce thyroid hormones. Secretion is stimulated by thyroid-releasing hormone from the hypothalamus. Free thyroid hormones (FT4 and FT3) work in a negative feedback loop to regulate the amount of TSH that is released from the pituitary gland. Dysregulation of TSH may either be a primary or secondary condition causing either hypo- or hyperthyroidism. Although thyroid issues are seen at all ages, onset is usually seen in the late 40s or early 50s, and affects females almost four times more often than males. An elevated TSH result usually means an underactive thyroid (hypothyroidism), whereas a decreased result means an overactive thyroid gland (hyperthyroidism) or excessive amounts of thyroid medication in an individual being treated for hypothyroidism. Measurement of serum TSH is the best initial laboratory test of thyroid function. If TSH levels are abnormal, free thyroxine (FT3) should then be measured. Also, measurement of anti-thyroperoxidase antibody should be done if the TSH level is high.