Lab Tests Reference Library

TSH is secreted by the pituitary gland to control thyroid hormone production. It is the single most sensitive test for thyroid dysfunction. High TSH indicates the pituitary is working harder to stimulate an underactive thyroid (hypothyroidism); low TSH indicates the pituitary is suppressed because the thyroid is overactive (hyperthyroidism) or the patient is receiving excess thyroid hormone.

Free T4 is the active, unbound form of thyroxine (T4) and represents only ~0.03% of total T4. Unlike total T4, free T4 is not affected by changes in binding proteins (TBG), making it a more accurate reflection of thyroid hormone status. Low Free T4 with high TSH confirms hypothyroidism; high Free T4 with low TSH confirms hyperthyroidism.

Free T3 is the most biologically active thyroid hormone — it exerts most of the metabolic effects at the cellular level. Most T3 is produced by conversion of T4 to T3 in peripheral tissues, not by direct thyroid secretion. Free T3 is measured when T3 toxicosis or sick euthyroid syndrome is suspected.

Anti-thyroid peroxidase (Anti-TPO) antibodies are autoantibodies that attack thyroid peroxidase, an enzyme essential for thyroid hormone synthesis. They are the most common thyroid autoantibodies and are the primary marker of autoimmune thyroid disease — particularly Hashimoto's thyroiditis and Graves' disease.

FSH is a gonadotropin hormone secreted by the anterior pituitary gland. In women, it stimulates ovarian follicle growth and estrogen production. In men, it stimulates Sertoli cells and is essential for spermatogenesis. FSH is critical for evaluating fertility, menstrual irregularities, and menopause.

LH is a gonadotropin hormone secreted by the anterior pituitary. In women, the LH surge triggers ovulation approximately 36–40 hours after the surge begins and stimulates progesterone production by the corpus luteum. In men, LH stimulates Leydig cells to produce testosterone. LH is a key fertility and menstrual cycle marker.

Prolactin is produced by the anterior pituitary and is the primary hormone responsible for milk production (lactation). Outside of pregnancy and breastfeeding, elevated prolactin (hyperprolactinemia) suppresses the hypothalamic-pituitary-gonadal axis, causing menstrual irregularities, infertility, and galactorrhea in women, and reduced libido, erectile dysfunction, and infertility in men.

Testosterone is the primary male sex hormone, produced mainly in the testes in men and in smaller amounts in the ovaries and adrenal glands in women. It is essential for male sexual development, muscle mass, bone density, red blood cell production, and libido. In women, elevated testosterone may indicate PCOS or adrenal disorders.

Estradiol (E2) is the most potent and predominant form of estrogen in reproductive-age women, produced primarily by ovarian follicles. It drives secondary sexual development, menstrual cycle regulation, bone maintenance, and cardiovascular protection. Estradiol is also present in men, derived from testosterone conversion in adipose tissue.

Cortisol is the primary glucocorticoid hormone produced by the adrenal cortex under the influence of ACTH from the pituitary gland. It regulates metabolism, immune response, blood pressure, and the stress response. Cortisol follows a clear circadian rhythm — peak levels are in the morning (7–9 AM) and lowest levels are at midnight.

AMH (Anti-Müllerian hormone) is secreted by granulosa cells of preantral and small antral follicles in the ovary. It directly reflects the size of the ovarian follicle pool and is the best serum marker of ovarian reserve. AMH levels decline progressively with age and reach undetectable levels after menopause. In men, AMH is produced by Sertoli cells in the testes and is used in pediatric endocrinology.