Human Hormones and Their Functions

Detailed view of neurons, showcasing synaptic connections involved in hormone signaling processes. — Hormones are signaling molecules that influence a wide range of physiological processes, and their imbalance could lead to many diseases, including diabetes and depression.
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Hormones coordinate different body functions by carrying messages through the bloodstream to target cells and tissues. Hormonal imbalances occur through inadequate or excess hormone synthesis, which could negatively affect an individual’s physical and mental well-being. In this article, explore different hormone types, functions, and mechanisms of action, as well as hormonal imbalance consequences and research on potential treatments.

What Are Hormones and Where Are They Made?

Hormones are messenger molecules synthesized by endocrine glands in response to specific internal or external stimuli.¹ Endocrine organs produce various hormones, facilitating diverse bodily functions. The major endocrine glands and their primary functions include the following.

Hypothalamus: Produces hormones that stimulate or inhibit the pituitary gland, such as gonadotropin-releasing hormone (GnRH), which triggers follicle-stimulating hormone (FSH) release; thyrotropin-releasing hormone (TRH), which leads to thyroid-stimulating hormone (TSH) release; and dopamine, which inhibits prolactin release.
Pituitary gland: Produces FSH, which leads to follicle development and sperm production; TSH, which triggers thyroid hormone (TH) release; oxytocin, which causes uterine contractions; and luteinizing hormone (LH), which stimulates estrogen and testosterone production.
Adrenal gland: Produces cortisol, which regulates carbohydrate, protein, and lipid metabolism and induces the body’s “fight or flight” stress response.
Gonads: Testes and ovaries produce testosterone, estrogen, and progesterone, which are involved in reproductive organ development, muscle and bone health, and secondary sex characteristics related to puberty.
Thyroid: Produces THs, including tri-iodothyronine (T3) and thyroxine (T4), which control metabolic processes in all cells.
Pancreas: Produces insulin and glucagon, which regulate carbohydrate metabolism.

The pituitary gland produces follicle stimulating hormone (FSH), luteinizing hormone (LH), thyroid stimulating hormone (TSH), oxytocin, and many other hormones, playing a central role in regulating when other endocrine organs produce and release hormones; the pineal gland produces melatonin and regulates circadian rhythms; the thymus produces hormones that regulate T-cell development and immunity; the pancreas produces insulin and glucagon, regulating glucose metabolism; the hypothalamus produces gonadotropin-releasing hormone (GnRH), thyrotropin-releasing hormone (TRH), and dopamine to regulate pituitary gland activity; the thyroid produces tri-iodothyronine (T3) and thyroxine (T4) and controls metabolic processes; the adrenal glands produce cortisol, regulating metabolism and stress responses; and the gonads produce androgens, estrogens, and progestogens involved in reproductive organ development, muscle and bone health, and secondary sex characteristics related to puberty. — The endocrine organs secrete various hormones, facilitating diverse bodily functions and regulating hormone production and release through positive and negative feedback loops.
Modified from © iStock, marrishuanna, Pacha M Vector

Hormone classification

Scientists classify hormones based on general molecular structure, size, and chemical properties. There are three main hormone classes: lipids (e.g., cortisol and estrogen), amines (e.g., dopamine and epinephrine), and peptides (e.g., adrenocorticotropic hormone and angiotensin).²

Lipid hormones are primarily steroid hormones synthesized from cholesterol in the adrenal gland and gonads.³ These hormones play essential roles in sexual differentiation, growth, and reproduction. Common steroid hormones include progestogens, glucocorticoids, androgens, and estrogens.

Amine hormones derive from modified amino acids, specifically tryptophan or tyrosine. For instance, tryptophan is a precursor to melatonin, an amine hormone produced by the pineal gland in the brain.⁴ This hormone regulates sleep-wake cycles, promotes immune responsiveness, and modulates aging.⁵ Another example of amine hormones are the thyroid hormones T3 and T4, which are iodine-containing tyrosine derivatives.⁶

Peptide hormones contain amino acid chains that vary in size, ranging from very short polypeptide chains such as oxytocin to larger proteins, including insulin and FSH.⁷ These hormones are crucial for regulating energy homeostasis, controlling appetite, and maintaining reproductive processes.

Hormone Mechanisms and Receptors

Hormones circulate in the blood independently or as complexes bound to specific plasma proteins such as globulins or albumin.⁸ When circulating hormones reach a target cell, they interact with specific receptors on the cell surface or within the cell and induce a series of biochemical reactions, also called a signaling cascade, which alters the cell’s activity.⁹

Hormone receptors contain binding sites that are highly complementary to specific hormone molecules. Water-soluble or hydrophilic hormones bind more readily to protein receptors embedded on the cell surface, while lipid-soluble hormones penetrate the target cell membrane and bind internal receptors.¹⁰

Steroid hormones and amine hormones interact with receptors in the cytoplasm or within the cell nucleus and form hormone-receptor complexes.⁹ Cytoplasmic complexes frequently translocate to the nucleus, where they interact with DNA and eventually regulate the activity of hormone-responsive genes.

In contrast, peptide hormones cannot enter cells and only bind to cell surface receptors.¹¹ The interaction between these hormones and their cell surface receptors may directly activate intracellular signaling pathways, influencing functions such as cellular metabolism.

After binding, hormones detach from the cell's receptor sites and enter the bloodstream, either unchanged or inactivated, and reach the liver or kidneys, where they are degraded or excreted through urine, respectively.

Hormone Regulation via Feedback Mechanisms

The body tightly regulates hormone production and release through feedback loops, which might involve more than one hormone to maintain homeostasis. Most hormones are regulated by negative feedback mechanisms as opposed to positive feedback mechanisms.

For example, in the negative feedback system that regulates glucagon and insulin secretion, a stimulus (e.g., high glucose) triggers hormone release into the blood.¹²When the blood glucose level reaches a predetermined threshold, it triggers insulin secretion from pancreatic cells, enhancing cellular glucose uptake and, thereby, lowering blood glucose amounts. In contrast, decreased blood sugar induces glucagon secretion, which stimulates glucose release from the liver, increasing blood glucose levels. This system helps maintain blood hormone concentrations within a narrow range.

Contrary to the negative feedback mechanism, positive mechanisms enhance the original stimuli instead of negating them.¹³ Although it is less common than negative feedback-based regulation, certain hormone levels are regulated through positive feedback loops. For example, during the menstrual cycle, estrogen levels temporarily increase, which triggers a further release of LH from the pituitary, eventually leading to ovulation. A positive feedback mechanism also regulates oxytocin release during childbirth. Uterine contraction during labor stimulates oxytocin release from the pituitary, and this cycle continues until the baby is born.

Hormone Imbalance and Treatment

Several diseases are associated with excessive or inadequate hormone secretion. A hormone imbalance could occur from disturbances at any point in the complex hormone-regulating feedback system or due to impaired endocrine gland development.¹⁴ For instance, aberrant cortisol, aldosterone, and dehydroepiandrosterone (DHEA) production may contribute to adrenal hormone imbalances that induce oxidative stress, damaging proteins, lipids, and DNA, and causing cellular dysfunction.²⁴ Hormone imbalances also result from genetic disorders, infections, or endocrine gland diseases, including tumors. For example, Cushing's disease results from a pituitary or other ectopic tumor that leads to excessive adrenocorticotrophic hormone (ACTH) production, causing high blood pressure and obesity.¹⁵

Finally, hormone imbalances can also arise during the aging process. Age-related biochemical changes may decrease endocrine system efficiency and target cell sensitivity.¹⁶ For instance, aging can lead to decreased melatonin levels, which may cause sleep disturbances, or reduced growth hormone (GH) levels, which may contribute to muscle loss and alter insulin sensitivity.^17-19

Table: Examples of endocrine disorders and their symptoms¹⁴

Endocrine Gland	Disease	Cause	Symptoms
Pituitary gland	Acromegaly	Excess GH	Broadening of hands, feet, and facial features, and headaches
Thyroid	Grave's disease	Excess TH	Goiter, fatigue, bulging eyes, skin thickening, and weight loss
Parathyroid	Hypoparathyroidism	Inadequate parathyroid hormone	Muscle cramp and tingling in the hands
Pancreas	Type 1 diabetes mellitus	Insufficient insulin	Polyuria, fatigue, and weight loss
Adrenal gland	Addison's disease	Insufficient cortisol	Hypotension, fatigue, abdominal pain, and nausea
Ovaries	Polycystic ovarian syndrome (PCOS)	Excess androgen	Irregular menstrual cycle, infertility, alopecia, and increased body and facial hair
Testes	Kallmann's syndrome	Insufficient androgen	Decreased libido, anosmia, and hearing loss

Clinicians diagnose endocrine disorders based on symptoms, physical examination, imaging tests, genetic profiling, and urine and blood analysis to determine anomalies in hormone levels or endocrine glands.²⁰ After diagnosis, they may treat the specific disorder via medications, chemotherapy, radiation therapy, or surgery, as required.

Scientists have developed therapeutics that target specific cell receptors by mimicking hormones to promote particular functions. For example, clinicians treat hypothyroidism with levothyroxine, which mimics the thyroxine hormone that is low in patients with the condition.²¹ Beta blockers are another common example of therapeutic agents that target hormone receptors. Physicians use beta blockers to treat patients with different cardiovascular conditions. These compounds are competitive antagonists that block adrenergic beta receptor sites, preventing catecholamine (adrenaline and noradrenaline) and angiotensin II signaling.²²

Scientists are also investigating dietary interventions for hormone imbalances, including nutritional antioxidants such as vitamin E, vitamin C, selenium, zinc, carotenoids, and probiotics, to potentially mitigate the adverse effects of adrenal hormone imbalances.²³

References