Hormone

A hormone is a signaling molecule secreted by an endocrine cell or gland that travels through the circulation to act on target cells bearing a specific receptor. Hormones are broadly grouped by chemical class into peptides and proteins (such as insulin, glucagon and oxytocin), steroids derived from cholesterol (such as cortisol, estradiol and testosterone) and tyrosine derivatives (such as thyroid hormones and catecholamines). Water-soluble peptide hormones generally cannot cross the cell membrane and act on cell-surface receptors, often G protein-coupled receptors or receptor tyrosine kinases, triggering rapid intracellular signaling. Lipid-soluble steroid hormones cross the membrane and bind intracellular receptors that regulate gene transcription.

Many of the first peptide medicines were human hormones or close copies of them. Insulin, introduced about a century ago, was the first, and hormone-derived drugs remain a large share of approved peptide therapeutics. Glucagon is used to treat severe hypoglycemia, and oxytocin is used in obstetrics. Peptide hormones are digested in the gut and cleared quickly from the blood, which is why most are given by injection.

Knowing whether a hormone is a peptide or a steroid explains much of how it behaves. Peptide hormones such as insulin and glucagon are chains of amino acids that dissolve in blood and bind receptors on the cell surface, producing fast signals inside the cell. Steroid hormones such as cortisol are made from cholesterol, pass through cell membranes and act mainly by changing which genes are switched on, so their effects tend to develop more slowly.

Hormones show how tightly the body balances opposing signals. Insulin lowers blood glucose after a meal, while glucagon from neighboring cells in the pancreas raises it during fasting by prompting the liver to release stored glucose. The two act as a feedback pair, and changing one hormone can shift a whole network of related signals, which is one reason hormone-based medicines are tested carefully in clinical trials and why taking unapproved hormone-like products without medical oversight carries real risk.

Hormone biology is the starting point for most peptide drug development. Researchers have repeatedly taken a natural hormone, identified the receptor it activates and then modified the sequence to make it last longer or act more selectively. GLP-1 receptor agonists and somatostatin analogues such as octreotide came from this approach. Knowing the parent hormone helps readers judge what a related drug is likely to do and which effects to expect.

“Peptide hormones and steroid hormones work the same way”

They differ in chemistry and in how they act. Peptide hormones are water-soluble and bind receptors on the cell surface, which trigger signaling cascades inside the cell. Steroid hormones are fat-soluble, cross the membrane and bind receptors inside the cell that regulate gene expression. These differences affect speed of action, duration and how each is given as a medicine.

“A hormone has only one job”

Most hormones act on several tissues. Oxytocin is best known for uterine contraction during labor and milk ejection during breastfeeding, but its receptors are also found in the kidney, heart, pancreas and brain. A hormone’s overall effect depends on which tissues carry its receptor and on other signals present at the time.

“Because hormones are natural, taking more is safe”

Being made by the body does not make extra amounts harmless. Hormone levels are tightly regulated, and excess insulin, for example, can cause dangerous hypoglycemia. Approved hormone medicines are prescribed and monitored for this reason. Unapproved peptide products sold as research chemicals have no such oversight, and their content and purity are not verified.

What is the difference between a peptide hormone and a steroid hormone?

A peptide hormone is a chain of amino acids, such as insulin, glucagon or oxytocin. A steroid hormone is built from cholesterol, such as cortisol or testosterone. Peptide hormones act on receptors at the cell surface, while steroid hormones enter cells and bind receptors that regulate gene activity.

Why are most peptide hormones given by injection?

Peptide hormones are broken down by digestive enzymes in the stomach and intestine, and they are poorly absorbed across the gut wall because of their size and water solubility. Injection bypasses the gut. Oral delivery of a few peptides has become possible only with specialized formulations, and even then only a small fraction of the peptide is absorbed.

Is insulin a hormone or a peptide?

Insulin is both. It is a hormone because it is secreted into the blood by pancreatic beta-cells and acts on distant tissues such as liver, muscle and fat. It is a peptide hormone because it is made of two amino acid chains joined by disulfide bonds. Insulin was the first hormone developed into a peptide medicine.