Hormone & endocrinology glossary

Hormone reports and health articles are full of technical terms. These plain-language definitions explain the words you are most likely to encounter when reading about hormone testing and treatment. For deeper reading, follow the links to our hormones, blood tests, and conditions sections.

ACTH (adrenocorticotropic hormone)

A pituitary hormone that signals the adrenal glands to make cortisol; part of the HPA axis.

Adrenal glands

Small glands above each kidney that produce cortisol, aldosterone, adrenaline, and some sex-hormone precursors.

Aldosterone

An adrenal hormone that helps regulate sodium, potassium, and blood pressure.

Androgen

A class of hormones, including testosterone, that drive male-typical characteristics and are present in everyone in differing amounts.

Assay

The laboratory method used to measure a substance, such as a hormone, in a sample. Different assays can give slightly different results and ranges.

Autoimmune

A process in which the immune system mistakenly targets the body's own tissue, as in Hashimoto's and Graves' disease.

Bioavailable hormone

The fraction of a hormone available to tissues — the free hormone plus the loosely (albumin-bound) portion.

Bioidentical

A term describing hormones with the same molecular structure as those the body makes; structure alone does not establish safety or effectiveness.

Calcitonin

A thyroid hormone involved in calcium regulation; also used as a tumor marker in certain thyroid cancers.

Cortisol

An adrenal steroid hormone central to metabolism and the stress response; it follows a daily (diurnal) rhythm.

DHEA / DHEA-S

An adrenal androgen precursor and its longer-lasting sulfated form, often measured to assess adrenal androgen output.

Diurnal rhythm

A daily cycle in a hormone's level, such as cortisol being higher in the morning and lower at night.

Endocrine system

The network of glands that produce hormones and release them into the bloodstream to regulate body functions.

Estradiol

The main and most potent estrogen during reproductive years, produced largely by the ovaries.

Estrogen

A group of hormones (estradiol, estrone, estriol) important in reproductive and many other body systems.

Euthyroid

A state of normal thyroid function, with thyroid hormone levels in the typical range.

Feedback loop

A regulatory cycle in which a hormone's level signals back to the gland controlling it, raising or lowering further production.

Follicle-stimulating hormone (FSH)

A pituitary hormone that supports egg and sperm development and signals the gonads.

Free hormone

The portion of a hormone not bound to carrier proteins, generally considered the biologically active fraction.

Glucagon

A pancreatic hormone that raises blood sugar, opposing insulin.

Gonads

The primary reproductive glands — the testes and ovaries — which produce sex hormones and gametes.

Hormone

A chemical messenger released by a gland into the blood to regulate the activity of cells and organs elsewhere.

HPA axis

The hypothalamus–pituitary–adrenal axis, the feedback system controlling cortisol.

HPG axis

The hypothalamus–pituitary–gonadal axis, the feedback system controlling sex hormones.

HPT axis

The hypothalamus–pituitary–thyroid axis, the feedback system controlling thyroid hormones.

Hyperthyroidism

An overactive thyroid producing too much thyroid hormone, speeding many body processes.

Hypogonadism

Reduced function of the gonads, leading to low sex-hormone levels.

Hypothalamus

A brain region that helps control the pituitary gland and, through it, much of the endocrine system.

Hypothyroidism

An underactive thyroid producing too little thyroid hormone, slowing many body processes.

IGF-1

Insulin-like growth factor 1, a hormone produced mainly in the liver that mediates many effects of growth hormone.

Insulin

A pancreatic hormone that lowers blood sugar by helping cells take up glucose.

Insulin resistance

A reduced response of cells to insulin, so more is needed to keep blood sugar in range.

Luteinizing hormone (LH)

A pituitary hormone that triggers ovulation and stimulates testosterone production.

Pituitary gland

A small gland at the base of the brain that releases hormones directing other glands.

Progesterone

A hormone important in the menstrual cycle and pregnancy, produced mainly after ovulation.

Prolactin

A pituitary hormone best known for supporting milk production; high levels can disrupt other hormones.

Reference range

The range of values considered typical for a test in a given population, set by the testing laboratory.

Reverse T3 (rT3)

An inactive form produced from T4; sometimes measured in specific clinical situations.

SHBG (sex hormone–binding globulin)

A protein that binds sex hormones in blood, affecting how much is free and active.

Steroid hormone

A hormone derived from cholesterol, including cortisol, aldosterone, and the sex hormones.

Subclinical

An early or mild pattern (e.g., subclinical hypothyroidism) with abnormal labs but few or no symptoms.

Thyroid gland

A butterfly-shaped gland in the neck that produces the thyroid hormones T4 and T3.

TSH (thyroid–stimulating hormone)

A pituitary hormone that signals the thyroid to make its hormones; commonly the first thyroid test ordered.

T3 (triiodothyronine)

The more active thyroid hormone, much of it converted from T4 in body tissues.

T4 (thyroxine)

The main hormone the thyroid releases; converted to the more active T3 as needed.

Vasopressin (ADH)

A pituitary hormone that helps the body conserve water by concentrating urine.