Polypeptide Hormones: Insulin, Growth Hormone, and the Endocrine System

Key Takeaways

• Structural class: Polypeptide hormones are amino acid chains that signal through cell-surface receptors — a mechanism categorically distinct from steroid hormones, which bind nuclear receptors.
• Major examples: Insulin (51 AA), growth hormone (191 AA), GLP-1 (30 AA), GIP (42 AA), peptide YY (36 AA), oxytocin (9 AA), vasopressin (9 AA) — each governing a different physiological domain.
• Incretin hormones: In healthy individuals, GLP-1 and GIP together are estimated to contribute approximately 50 to 70% of postprandial insulin secretion across published studies, with the effect substantially diminished in type 2 diabetes; they are the pharmacological targets of semaglutide, liraglutide, and tirzepatide.
• Measurable via bloodwork: Fasting insulin, IGF-1, HbA1c, and TSH are standard clinical markers that directly or indirectly reflect polypeptide hormone activity.
• Clinical relevance: As of April 2026, polypeptide hormones or their synthetic analogs account for several of the most widely prescribed drug classes in current U.S. medicine, including GLP-1 receptor agonists and insulin formulations.

The hormones that regulate how your body handles glucose after a meal, how quickly you grow in childhood, how much you eat at dinner, and how strongly you bond with people you trust — these are all polypeptides. They are not the same class of molecule as testosterone or estrogen. They work through fundamentally different chemistry and different cellular machinery.

This article covers the major polypeptide hormones: what they are structurally, where they are made, how they signal cells, and what they do in the body. It also covers which bloodwork markers reflect their activity and why those markers matter clinically.

What Polypeptide Hormones Are

A polypeptide hormone is a chain of amino acids that functions as a signaling molecule in the endocrine system. These hormones bind to receptors on the surface of target cells and activate intracellular cascades rather than entering cells themselves — a mechanism that distinguishes them from steroid hormones, which are lipid-derived and cross the cell membrane directly. Forbes and colleagues, in the StatPearls Biochemistry, Peptide chapter, described polypeptide hormones as among the most structurally and functionally diverse endocrine regulators in the human body. Their diversity arises directly from the diversity of amino acid sequences — different sequences produce different three-dimensional structures, which bind different receptors and trigger different downstream responses.

Discovery and historical context

The clinical story of polypeptide hormones begins with insulin. Frederick Banting and Charles Best isolated insulin from dog pancreas in 1921; the first human patient (Leonard Thompson) was treated in January 1922, establishing insulin as the first polypeptide hormone therapy in medicine. Growth hormone was characterized through the mid-20th century, with recombinant production enabling the modern therapeutic era. The incretin hormones GLP-1 and GIP were identified and characterized in the 1970s through 1990s, a lineage that directly produced today's most commercially significant polypeptide drugs: GLP-1 receptor agonists. Ørskov, in a 1992 review in Diabetologia, published an early characterization of GLP-1 as a new hormone of the entero-insular axis, the foundational paper in the intellectual lineage that culminated in semaglutide and tirzepatide.

How Polypeptide Hormones Signal Cells

Polypeptide hormones are water-soluble and cannot cross the lipid bilayer of the cell membrane. Their signaling mechanism therefore operates at the cell surface: the hormone binds a transmembrane receptor, the receptor changes conformation, and an intracellular cascade transmits the signal to the cell's machinery.

G protein-coupled receptor signaling

Most polypeptide hormones signal through G protein-coupled receptors (GPCRs), which represent the largest receptor superfamily in the human genome. GPCR activation leads to activation or inhibition of adenylyl cyclase (producing cAMP as a second messenger), phospholipase C (producing IP3 and DAG), or other downstream effectors depending on the G-protein subtype involved. Posner, in a 2010 review in Progress in Brain Research, described how peptide hormones and growth factors activate cell-surface receptors to drive metabolic and proliferative responses through kinase cascades and transcription-factor activation. GLP-1 is a prototypical example: it signals through a GPCR on pancreatic beta cells, activating cAMP-PKA pathways that potentiate glucose-stimulated insulin secretion. Abid and colleagues, in a 2021 paper in ACS Chemical Biology, reviewed the challenge of identifying receptors for neuropeptides and peptide hormones, highlighting that receptor identification is a prerequisite for therapeutic targeting.

Receptor tyrosine kinase signaling

Insulin signals through a receptor tyrosine kinase (RTK) rather than a GPCR — the insulin receptor has intrinsic kinase activity that is activated by insulin binding. Boucher and colleagues, in a 2014 review in Cold Spring Harbor Perspectives in Biology, described the insulin receptor signal transduction network in molecular detail, covering the PI3K/Akt pathway (which governs glucose uptake and anabolic metabolism) and the MAPK pathway (which regulates cell growth and differentiation). The insulin receptor superfamily also includes IGF-1 receptors, which are structurally similar and share signaling components — explaining why insulin and IGF-1 have overlapping metabolic and anabolic effects.

Polypeptide hormone synthesis and processing

Polypeptide hormones are synthesized as longer precursor molecules (preprohormones) that undergo proteolytic cleavage to produce the active hormone. Insulin is synthesized as preproinsulin, cleaved to proinsulin, then cleaved again to produce the A and B chains of active insulin plus C-peptide. This processing pathway is physiologically significant: C-peptide measurement distinguishes endogenous insulin production from exogenous insulin administration. Most polypeptide hormones follow this general prepropeptide processing pattern. Nauck and colleagues, in a 2018 review in Diabetes, Obesity and Metabolism, reviewed incretin hormone biology including GLP-1 and GIP processing from their larger precursor molecules, noting that DPP-IV cleavage rapidly inactivates circulating GLP-1 — a finding that drove the development of DPP-IV inhibitors as a drug class.

Major Polypeptide Hormones in Human Biology

The human endocrine system depends on dozens of polypeptide hormones. The most clinically relevant examples span the pancreas, pituitary, gut, and hypothalamus.

Insulin: blood glucose regulation

Insulin is a 51-amino acid polypeptide produced by beta cells in the pancreatic islets of Langerhans. It consists of an A chain (21 amino acids) and a B chain (30 amino acids) joined by two inter-chain disulfide bridges, with a third disulfide bond within the A chain. Mayer and colleagues, writing in Biopolymers in 2007, described insulin's two-chain architecture and disulfide configuration, which create the precise surface geometry required for receptor binding and subsequent glucose disposal signaling. Rahman and colleagues, in a comprehensive 2021 review in the International Journal of Molecular Sciences, detailed insulin's role in health and disease, covering glucose regulation, anabolic signaling (protein synthesis, glycogen storage, lipogenesis), and the molecular mechanisms underlying insulin resistance. Shabanpoor and colleagues, in a 2009 review in Vitamins and Hormones, placed insulin in the broader context of the human insulin superfamily, which includes IGF-1, relaxin, and several other structurally related polypeptides. Fasting insulin is the most direct clinical measure of this hormone's activity at baseline.

Growth hormone: cellular growth and tissue repair

Growth hormone is a 191-amino acid polypeptide produced by somatotroph cells in the anterior pituitary. Sami and colleagues, writing in Current Protein & Peptide Science in 2007, described the structure-function relation of somatotropin: four alpha helices arranged anti-parallel, two disulfide bridges, and a molecular architecture that enables engagement with two GH receptor molecules simultaneously — a structural feature central to its receptor activation mechanism. Aghili and colleagues, in their 2025 Molecular Biology Reports paper, reviewed GH signaling pathways and clinical implications, covering its direct effects on metabolism (lipolysis, glucose counter-regulation) and its indirect effects via IGF-1, a 70-amino acid polypeptide produced primarily in the liver in response to GH stimulation. Laron, writing in Molecular Pathology in 2001, established IGF-1 as a major mediator of GH's anabolic effects. Since GH itself circulates in brief pulsatile bursts, IGF-1 is the practical clinical measure for assessing GH axis activity. Reh and colleagues, in a 2010 review in Clinical Pharmacology: Advances and Applications, reviewed recombinant somatotropin therapy for GH deficiency and Turner syndrome.

GLP-1 and GIP: the incretin hormones

GLP-1 (glucagon-like peptide-1) is a 30-amino acid hormone produced by L-cells in the distal small intestine and colon in response to nutrient ingestion. GIP (glucose-dependent insulinotropic polypeptide) is a 42-amino acid hormone produced by K-cells in the duodenum and proximal jejunum. Together, they constitute the incretin hormone system. Baggio and colleagues, in a landmark 2007 review in Gastroenterology, provided a comprehensive account of incretin biology, establishing that, in healthy individuals, incretins are estimated to contribute approximately 50 to 70% of postprandial insulin secretion, with this effect substantially diminished in type 2 diabetes — the incretin effect. Christensen and colleagues, in a 2016 paper in the Danish Medical Journal, examined GIP effects on insulin and glucagon secretion in humans. GLP-1 also slows gastric emptying and promotes satiety through hypothalamic pathways; Abu-Hamdah and colleagues, in a 2009 clinical review in JCEM, documented GLP-1's extrapancreatic effects on the heart, brain, and gut, establishing why the GLP-1 receptor agonist drug class produces effects beyond glycemic control. As of April 2026, both GLP-1 and GIP biology are the pharmacological basis for tirzepatide (marketed as Mounjaro and Zepbound), a dual GIP/GLP-1 receptor co-agonist FDA-approved for type 2 diabetes (as Mounjaro), chronic weight management in adults with obesity or overweight with a weight-related comorbidity, and moderate-to-severe obstructive sleep apnea in adults with obesity (as Zepbound). Compounded tirzepatide is not FDA-approved and is not interchangeable with the branded products.

Brand-name references above identify FDA-approved manufacturer products. Patients considering any GLP-1 or GIP/GLP-1 receptor agonist therapy should review full prescribing information with a licensed clinician; this article is educational and does not constitute prescribing guidance. Superpower does not sell any of the named branded products and this article should not be read as endorsing, comparing, or recommending any specific prescription medication.

Peptide YY (PYY): satiety signaling

PYY is a 36-amino acid gut hormone produced by L-cells in the distal small intestine and colon — the same cells that produce GLP-1. It is released proportionally to meal energy content and acts on hypothalamic Y2 receptors to suppress appetite. Batterham and colleagues published a landmark 2002 study in Nature showing that a 90-minute peripheral intravenous infusion of PYY3-36 reduced food intake by approximately 33% over the subsequent 24 hours in healthy volunteers — a landmark demonstration of PYY's satiety role, though the magnitude of this effect in subsequent trials has varied. le Roux and colleagues reported in Endocrinology in 2006 that individuals with obesity have attenuated postprandial PYY responses and reduced satiety, contributing to higher food intake. Boey and colleagues, writing in Neuropeptides in 2008, showed that PYY transgenic mice are protected against diet-induced and genetic obesity, providing preclinical genetic evidence, in a mouse model, that elevated PYY is sufficient to resist weight gain.

Oxytocin and vasopressin: neuropeptide hormones

Oxytocin and vasopressin (antidiuretic hormone, ADH) are both 9-amino acid polypeptides produced in hypothalamic nuclei and released by the posterior pituitary. They are among the most evolutionarily conserved polypeptide hormones known. Mohr and colleagues, in a 1995 review in Vitamins and Hormones, traced their evolutionary conservation across vertebrates, noting their structural near-identity and divergent receptor specificity as a model of how small amino acid differences in a polypeptide can produce dramatically different biological functions. Baribeau and colleagues, in a 2015 review in Frontiers in Neuroscience, described how oxytocin and vasopressin link pituitary neuropeptides to social neurocircuitry governing bonding and affiliation. Rigney and colleagues, in a 2022 review in Endocrinology, reviewed the roles of oxytocin and vasopressin in social behavior and described emerging therapeutic interest in modulating these polypeptide systems. Perisic and colleagues, in a 2024 review in Trends in Biochemical Sciences, updated oxytocin and vasopressin signaling in health and disease.

Polypeptide Hormones vs. Steroid Hormones

The most fundamental distinction between polypeptide and steroid hormones is their mechanism of cellular action. Polypeptide hormones bind surface receptors and signal through second messengers; steroid hormones cross the cell membrane and directly alter gene expression through nuclear receptors. Every downstream consequence of that difference — speed of action, duration of effect, receptor pharmacology, drug design strategy — follows from this structural and mechanistic divergence.

• Chemical identity:
  • Polypeptide hormones: chains of amino acids; water-soluble; cannot cross the cell membrane
  • Steroid hormones: cholesterol-derived lipids; fat-soluble; cross the cell membrane freely
• Receptor location:
  • Polypeptide hormones: cell-surface receptors (GPCRs, RTKs)
  • Steroid hormones: intracellular and nuclear receptors
• Signal mechanism:
  • Polypeptide hormones: second messengers (cAMP, IP3), kinase cascades, phosphorylation events
  • Steroid hormones: direct DNA binding, transcription factor activity, altered gene expression
• Speed of action:
  • Polypeptide hormones: seconds to minutes (insulin response begins within seconds of glucose spike)
  • Steroid hormones: hours (gene expression changes require transcription and translation)
• Half-life:
  • Polypeptide hormones: typically minutes (endogenous GLP-1: 1 to 2 minutes; endogenous GH: approximately 15 to 20 minutes)
  • Steroid hormones: typically hours to days
• Examples:
  • Polypeptide hormones: insulin, growth hormone, GLP-1, GIP, PYY, oxytocin, vasopressin, glucagon
  • Steroid hormones: testosterone, estradiol, cortisol, aldosterone, DHEA

This distinction matters practically when evaluating drug classes. GLP-1 receptor agonists work because GLP-1 is a polypeptide hormone with a defined surface receptor — drug design targets that receptor with a more stable analog. Testosterone replacement therapy works because testosterone is a steroid with a nuclear receptor — drug design delivers the steroid directly. The two categories require entirely different pharmacological approaches, and conflating them (as consumer marketing sometimes does) produces confusion about mechanism, safety, and monitoring requirements.

Why This Matters for Your Health

Polypeptide hormone biology is directly measurable through standard bloodwork. Fasting insulin captures the activity of a clinically central endogenous polypeptide hormone, and elevated fasting insulin has been described in longitudinal metabolic research, including the Whitehall II cohort analysis by Tabák and colleagues in The Lancet, as an early measurable change that can precede fasting glucose elevation in the progression toward insulin resistance and type 2 diabetes. IGF-1 reflects integrated GH secretion and provides an objective window into the growth hormone axis. Hemoglobin A1c captures three months of incretin polypeptide hormone function as it manifests in average blood glucose. TSH reflects hypothalamic TRH polypeptide activity driving the pituitary-thyroid axis. These are not abstract measures — they are direct readouts of polypeptide hormone systems operating in the body.

Understanding the biology behind these markers is what makes the numbers interpretable rather than arbitrary. That translation from molecular biology to actionable data is at the core of Superpower's approach to preventive health.

Which Biomarkers Are Relevant if You Are Exploring Polypeptide Hormone Biology?

The major polypeptide hormones have corresponding clinical markers that make their activity measurable. Establishing these baselines provides objective reference points that make any subsequent biological change interpretable.

• Fasting insulin: A direct measure of a clinically central polypeptide hormone. In the Whitehall II cohort, Tabák and colleagues traced rising insulin resistance for up to 13 years before type 2 diabetes diagnosis, and elevated fasting insulin can precede overt disease by years. Standard component of a comprehensive metabolic panel.
• IGF-1: The primary clinical surrogate for growth hormone axis activity. Since endogenous GH is secreted in pulses, a single GH measurement is not informative; IGF-1 integrates GH secretion over days and is the standard monitoring marker for the GH axis in both deficiency and excess states.
• Hemoglobin A1c (HbA1c): The primary endpoint in all major incretin polypeptide hormone drug trials. A baseline HbA1c characterizes the integrated glycemic consequence of insulin and incretin polypeptide function over the prior three months.
• Fasting glucose: Foundational metabolic marker. Polypeptide hormones governing glucose — insulin, glucagon, GLP-1, GIP — collectively determine fasting glucose levels. Elevated fasting glucose indicates a failure of this polypeptide system to maintain euglycemia.
• Thyroid-stimulating hormone (TSH): Reflects hypothalamic TRH (a tripeptide) driving pituitary secretion of the glycoprotein hormone TSH, which governs thyroid function. TSH is the most sensitive available marker for hypothalamic-pituitary-thyroid axis activity.
• Triglycerides: Tirzepatide, a dual GIP/GLP-1 receptor agonist, was associated in the SURPASS 1–4 phase 3 program with improvements in cardiometabolic risk factors including triglycerides. Baseline triglycerides provide the lipid context for metabolic polypeptide hormone function.
• Free T3 (triiodothyronine): The active thyroid hormone downstream of TSH and thyroid polypeptide signaling. Free T3 reflects the peripheral conversion of T4 to the active form and completes the picture of hypothalamic-pituitary-thyroid polypeptide axis function.

A comprehensive assessment of polypeptide hormone health begins with these markers. Superpower's metabolic health panel covers the insulin-glucose-HbA1c cluster, providing the interpretive framework that makes these numbers clinically useful rather than descriptive.