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What Is HGH? Understanding Human Growth Hormone

Dec 11, 2023
4 min read

Updated: Oct 12

By HGHThailand Team

Human growth hormone (HGH), also called somatotropin or simply growth hormone (GH), is a protein hormone produced by the pituitary gland. It drives growth in childhood and, in adults, plays a role in metabolism, tissue repair, and the balance of fat and muscle. This article explains what HGH is, how it works, and how its levels change over a lifetime.

What Are Hormones?

To understand where HGH fits, it helps to start with hormones in general. Hormones are molecules secreted by glands in the body that act as chemical messengers, carrying regulatory signals through the bloodstream.

Hormones are part of the endocrine system, which is made up of various glands that secrete them. These messengers carry specific signals to target receptors, triggering a specific response. Hormones fall into three main chemical classes: steroid hormones, protein (or polypeptide) hormones, and amine hormones.

  • Steroid hormones are derived from cholesterol and, because of their structure, can pass through cell membranes.

  • Protein (polypeptide) hormones are built from amino acids and are typically water-soluble, so they cannot pass through cell membranes.

  • Amine hormones are derived from the amino acid tyrosine and contain an amine group.

HGH belongs to the protein class, which is why it cannot be taken orally in its active form: the digestive tract breaks it down. For this reason most peptide hormones have negligible oral bioavailability, and an effective dose cannot be delivered as a pill [1].

What Is HGH and Where Is It Made?

HGH is a protein hormone composed of 191 amino acids. It is produced and secreted by somatotropic cells in the anterior pituitary gland.

Its production is regulated by a system of feedback mechanisms that respond to factors such as sleep, nutrition, stress, exercise, and growth hormone itself. HGH is not released at a constant rate; it is secreted in pulses. On average, it is secreted every 3–5 hours, and the largest pulse follows sleep onset, and most of the day's release occurs during the first hours of sleep [2][3].

This pulsatile pattern is central to how the hormone works, because it lets HGH act on different tissues at different times.

HGH Mechanism of Action

HGH has several important functions. It promotes growth and development, particularly of bones and muscles, and it also plays a role in fat metabolism and skin regeneration.

One key mechanism is direct: HGH attaches to target cells and triggers a response, which can include changes in muscle and bone tissue.

The second mechanism is indirect, through the liver. When HGH binds to receptors on liver cells, it triggers the release of insulin-like growth factor 1 (IGF-1), a protein hormone structurally similar to insulin [4][5]. IGF-1 supports the uptake of amino acids by muscle fibers and other target cells, an effect that is more pronounced after a meal, when glucose and amino acid levels in the blood are higher. Through this pathway, HGH helps direct available nutrients toward protein synthesis and cell growth.

Together, these direct and indirect pathways let HGH act on bone, muscle, fat, and the liver through a single signaling system.

Do HGH Levels Decline With Age?

Yes. In general, HGH levels rise during childhood, peak during puberty, and decline as we get older [4].

A decline in growth hormone is a normal part of aging. In some cases, a clinically significant deficiency can occur, and that is a medical diagnosis made by a doctor using blood tests, not something to self-assess or self-treat.

Diagram with three cards on what shapes HGH levels: sleep drives the pulse, levels fall with age, and diet and activity matter.

How HGH Relates to the Body's Other Anabolic Hormones

HGH, testosterone, and insulin are often described as the body's primary anabolic hormones. Understanding how they interact is useful context.

Testosterone

HGH has been reported to influence testosterone levels, and both hormones respond to exercise as a stimulus. Both play a role in protein synthesis, which is essential for muscle growth and repair.

The testes are the primary source of testosterone in men, with smaller amounts produced in women. It supports the development of male characteristics and plays a key role in protein synthesis.

Both hormones support protein synthesis and muscle maintenance, though the exact nature of their interaction is still being studied [5].

Insulin

Insulin and HGH both help regulate metabolism. Their effects are influenced by diet, especially carbohydrate intake.

The pancreas releases insulin mainly in response to eating carbohydrates, particularly simple carbs. Frequent carb-heavy meals cause repeated insulin spikes, and excess carbohydrate can be stored as fat.

HGH release is also influenced by nutrition. The GH/IGF-1 axis responds to changes in energy and nutrient intake, a relationship studied for its role in metabolism and disease risk [6].

Conclusion

HGH is a protein hormone produced by the pituitary gland that supports growth, metabolism, and tissue repair, acting directly and through IGF-1. Levels naturally decline with age. Healthy sleep, regular training, and balanced nutrition are associated with healthy natural hormone function [2][3][5].

This article is informational only and is not medical advice. Talk to a qualified healthcare professional about any hormone concern.

Sources

  1. Craik DJ, Fairlie DP, Liras S, Price D. The future of peptide-based drugs. *Chemical Biology & Drug Design*. 2013;81(1):136–147. PMID 23253135. https://pubmed.ncbi.nlm.nih.gov/23253135/

  2. Van Cauter E, et al. Physiology of growth hormone secretion during sleep. *The Journal of Pediatrics*. 1996;128(5 Pt 2):S32–37. PMID 8627466. https://pubmed.ncbi.nlm.nih.gov/8627466/

  3. Van Cauter E, Spiegel K, Tasali E, Leproult R. Metabolic consequences of sleep and sleep loss. *Sleep Medicine*. 2008;9 Suppl 1:S23–28. PMID 18929315. https://pubmed.ncbi.nlm.nih.gov/18929315/

  4. Endocrine Society. Hormones and Aging: An Endocrine Society Scientific Statement. *The Journal of Clinical Endocrinology & Metabolism*. 2023;108(8):1835. PMID 37326526. https://pubmed.ncbi.nlm.nih.gov/37326526/

  5. Kraemer WJ, et al. Growth Hormone(s), Testosterone, Insulin-Like Growth Factors, and Cortisol: Roles and Integration for Cellular Development and Growth With Exercise. *Frontiers in Endocrinology*. 2020;11:33. PMID 32158429. https://pubmed.ncbi.nlm.nih.gov/32158429/

  6. Fanti M, Longo VD. Nutrition, GH/IGF-1 signaling, and cancer. *Endocrine-Related Cancer*. 2024;31(11):e230048. PMID 39166749. https://pubmed.ncbi.nlm.nih.gov/39166749/ ---

 
 
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