Hormone Metabolism

Introduction: Hormone metabolism describes how chemical messengers are made, transported, activated, and cleared to regulate energy use, growth, and internal balance. This article explains the main chemical classes of hormones, how peptide, steroid, and amino-acid-derived hormones are synthesized, and why most circulate bound to plasma proteins with very different half-lives. It outlines core metabolic regulators, insulin and glucagon for blood glucose, thyroid hormones T3 and T4 for basal metabolic rate, and cortisol for glucose production, protein breakdown, and fat redistribution, and shows how negative feedback loops keep their levels within narrow ranges. It also covers peripheral activation steps, including deiodinase conversion of T4 to T3 and the role of selenium-dependent enzymes.

How Hormones Are Made, Classified, and Carried

Hormones are grouped by chemistry, which determines how they are made and how they travel.

The word hormone comes from the Greek hormao meaning I excite or arouse, and hormones exert effects by binding specific receptors after reaching target cells. Their functions fall into broad categories including reproduction and sexual differentiation, development and growth, maintenance of the internal environment, and regulation of metabolism and nutrient supply. A single hormone can affect more than one function, and each function is usually controlled by several hormones. Secretion is kept in balance by negative feedback control, where rising hormone levels act back on the hypothalamus and pituitary or on the gland itself to reduce further release.

Hormones are derived from three chemical starting points. The most numerous are protein or peptide hormones, ranging from three to over 200 amino acids, which are made by transcription of a gene into mRNA, translation on rough endoplasmic reticulum as a pre-prohormone, cleavage of the signal sequence, and further processing in the Golgi into secretory granules. Steroid hormones, including cortisol and sex steroids, are derived from cholesterol in mitochondria and smooth endoplasmic reticulum, and their synthesis does not require immediate gene expression but depends on specific enzymes expressed in each steroidogenic cell. A third group comes from single amino acids, tyrosine yields catecholamines and thyroid hormones, while tryptophan yields serotonin and melatonin, and phospholipids yield eicosanoids.

Once released, transport differs by solubility. Steroid and thyroid hormones are poorly soluble in water and circulate with more than 90 percent bound to specific plasma globulins or albumin, with only the free fraction considered biologically active. Binding proteins are made mainly in the liver and create a circulating reservoir that slows metabolism. Half-lives reflect this chemistry, catecholamines from the adrenal medulla last seconds, protein and peptide hormones last minutes, while steroid and thyroid hormones last hours.

• Peptide hormones: synthesized as large precursors, stored in granules, act via cell surface receptors.
• Steroid hormones: made from cholesterol, diffuse across membranes, act mainly via intracellular receptors after release.
• Amine and thyroid hormones: derived from tyrosine, thyroid hormones bind plasma proteins extensively and act like steroids inside the nucleus.

Key Hormones That Control Metabolic Rate and Fuel Use

Insulin, glucagon, thyroid hormone, and cortisol work together to balance fuel storage and use.

Blood glucose varies across feeding and fasting, and insulin and glucagon from the pancreas are the two hormones primarily responsible for maintaining homeostasis. Insulin is produced by pancreatic beta cells when glucose rises, it lowers blood glucose by increasing glucose uptake into target cells and by stimulating the liver to convert glucose to glycogen for storage. Glucagon is released when glucose falls, it raises blood glucose by stimulating glycogen breakdown in liver and muscle and by promoting gluconeogenesis from amino acids in the liver. Rising glucose inhibits further glucagon release, while falling glucose reduces insulin, forming paired negative feedback loops.

Thyroid hormone controls basal metabolism and growth through the hypothalamic-pituitary-thyroid axis. The thyroid gland releases mostly thyroxine (T4) and smaller amounts of triiodothyronine (T3). In target tissues, T4 is converted to active T3 by type I and type II deiodinases, while type III deiodinase makes inactive reverse T3. Thyroid hormone increases basal metabolic rate in part by increasing gene expression of Na+/K+ ATPase, leading to greater oxygen consumption and heat production. It stimulates carbohydrate metabolism, promotes protein anabolism at physiological doses, and can increase lipolysis or lipid synthesis depending on metabolic status. It does not directly set blood glucose but increases glucose reabsorption, gluconeogenesis, and glycogen synthesis.

Cortisol, the principal glucocorticoid from the adrenal zona fasciculata, is regulated by the hypothalamic-pituitary-adrenal axis and influences metabolism during stress and fasting. It elevates blood glucose by enhancing hepatic gluconeogenesis through activation of glucose-6-phosphatase and phosphoenolpyruvate carboxykinase, using amino acids from muscle proteolysis and fatty acids from lipolysis as substrates, while reducing glucose uptake in skeletal muscle and adipose tissue. In muscle, cortisol stimulates protein breakdown via the ubiquitin-proteasome system and suppresses protein synthesis by inhibiting mTOR signaling. In adipose tissue, cortisol promotes both differentiation and lipolysis, and sustained elevation increases visceral fat partly through local activation of cortisone to cortisol by 11-beta-hydroxysteroid dehydrogenase type 1.

Peripheral activation depends on micronutrients. Selenium is a cofactor for enzymes that convert inactive thyroid hormone to active T3 in cells, so low selenium can produce signs similar to iodine deficiency. Selenium is incorporated into at least 25 selenoproteins involved in thyroid hormone metabolism, antioxidant defense, and immune function.

• Insulin-glucagon pair: opposite actions on glucose uptake, glycogen storage, glycogenolysis, and gluconeogenesis maintain blood glucose within a narrow range.
• Thyroid activation: deiodinases in liver, kidney, muscle, and brain determine local T3 availability and metabolic rate.
• Cortisol modulation: permissive effects on glucagon and catecholamines amplify glucose production during stress while conserving fuel for the brain.

Related Topics

• Principles of Endocrinology
• Hormonal Regulation of Glucose Metabolism
• Thyroid Hormone Physiology
• Cortisol and Stress Metabolism
• Selenium in Thyroid Hormone Activation

📌 Frequently Asked Questions

Why do some hormones work in seconds and others take hours?

Water-soluble hormones like catecholamines and peptides bind cell surface receptors and trigger rapid second messenger cascades, and they are cleared quickly, giving half-lives of seconds to minutes. Lipid-soluble steroid and thyroid hormones travel bound to plasma proteins, enter cells, and change gene transcription, which takes longer and gives half-lives of hours.

How does the body turn inactive thyroid hormone into active hormone?

The thyroid releases mostly T4. In peripheral tissues, type I and type II deiodinase enzymes remove one iodine to form T3, the active form. These deiodinases contain selenium, so adequate selenium intake supports normal thyroid hormone activation.

References

1. Endocrinology, NCBI Bookshelf. (Principles of endocrinology). Classification, synthesis, transport, and feedback of hormones.
2. LibreTexts Chemistry. (2023). Hormonal Regulation of Metabolism. Insulin and glucagon regulation of blood glucose.
3. StatPearls, NCBI Bookshelf. Physiology, Thyroid Hormone. T3/T4 synthesis, deiodinases, metabolic rate effects.
4. StatPearls, NCBI Bookshelf. Physiology, Cortisol. HPA axis, gluconeogenesis, protein catabolism, adipose effects.
5. LibreTexts Medicine. Selenium. Role in thyroid hormone metabolism and deiodinase function.