Growth hormone (GH), also known as somatotropin, is a single-chain peptide hormone that plays a widespread role in regulating the human body. It is synthesized and secreted by specialized cells called somatotrophs, which are located within the anterior lobe of the pituitary gland. GH is released into the bloodstream and acts on nearly every tissue in the body, coordinating complex processes related to growth, development, and energy balance. Although most recognized for its effects on height during youth, GH maintains a continuous influence on body structure and metabolism throughout a person’s entire life.
How Growth Hormone Signals in the Body
Growth hormone exerts its effects through a sophisticated communication network known as the Growth Hormone–Insulin-like Growth Factor 1 (GH-IGF-1) axis. Once released from the pituitary gland, GH travels through the circulation and binds to specific receptors on the surface of target cells. Binding to these receptors initiates a signaling cascade inside the cell, most notably activating the JAK-STAT pathway, which alters gene transcription.
A significant portion of GH’s action is indirect, relying on the production of a second hormone, Insulin-like Growth Factor 1 (IGF-1). The liver is the primary site where GH stimulation triggers a substantial release of IGF-1 into the bloodstream. This IGF-1 then acts on distant tissues, mediating many of the hormone’s growth-promoting effects.
GH also has direct effects on certain target cells, independent of IGF-1. For example, it can directly influence fat cells to promote the breakdown of stored triglycerides. The overall biological action of GH is a combination of its immediate, direct actions and the prolonged effects mediated by IGF-1. This dual mechanism allows for both rapid metabolic adjustments and sustained anabolic processes.
The Primary Role in Skeletal and Tissue Growth
The most visible function of growth hormone is its regulation of longitudinal growth in children and adolescents. GH stimulates the proliferation of chondrocytes, which are the cartilage cells found within the epiphyseal plates, or growth plates, of long bones. The multiplication and subsequent maturation of these cells cause the bones to elongate, resulting in an increase in height. This process continues until the end of puberty when the growth plates fuse into solid bone.
Beyond height, GH also promotes the overall building and repair of soft tissues throughout the body. It stimulates the uptake of amino acids by muscle cells, which are the building blocks necessary for protein synthesis. This anabolic effect leads to an increase in muscle mass and the regeneration of connective tissues. Even after linear growth stops, this function remains active, contributing to the maintenance of lean body mass in adulthood.
Influence on Metabolism and Body Composition
Growth hormone acts as a regulator of the body’s energy balance, adjusting how the body manages carbohydrates, fats, and proteins. In terms of glucose metabolism, GH displays an anti-insulin effect, meaning it tends to raise blood sugar levels. This is partly achieved by promoting gluconeogenesis, the process where the liver creates new glucose from non-carbohydrate sources. GH also reduces the sensitivity of peripheral tissues, such as muscle, to the action of insulin, ensuring glucose remains available for the brain and other processes.
GH is also a lipolytic agent, influencing the breakdown of fat stores. It stimulates the release of fatty acids from adipose tissue (body fat) into the bloodstream, providing an alternative fuel source for many organs. This action helps to shift the body’s energy source away from glucose and toward fat utilization. By increasing protein synthesis and promoting lipolysis, GH helps to shape body composition by favoring lean muscle mass over fat storage.
Factors Affecting Growth Hormone Secretion
The release of growth hormone from the pituitary gland is not continuous but occurs in a pulsatile manner, meaning it is secreted in bursts throughout the day. The timing and size of these pulses are tightly controlled by two hormones from the hypothalamus in the brain: Growth Hormone-Releasing Hormone (GHRH) stimulates release, while somatostatin inhibits it. The balance between these two regulatory signals determines the overall GH output.
One of the largest surges in GH release occurs shortly after the onset of deep, non-rapid eye movement sleep. Physical exercise is another physiological stimulus that can trigger a temporary increase in GH levels. Nutritional status also plays a role; fasting or periods of low blood sugar tend to stimulate GH release, while a meal rich in carbohydrates or a state of high blood sugar will suppress its secretion. Stress and certain amino acids can also influence the hypothalamic signals, modulating the pattern of GH pulsatility.
Conditions Related to Abnormal Levels
Disruptions in the precise regulation of growth hormone can lead to distinct physiological conditions, depending on the timing and nature of the imbalance. If there is an excess of GH secretion, known as hypersecretion, the resulting condition depends on the age of the individual. Hypersecretion occurring in childhood, before the growth plates have fused, leads to gigantism, characterized by excessive height and overall body size.
If GH hypersecretion begins during adulthood, after the growth plates have closed, the condition is called acromegaly. Instead of height increase, the excess hormone causes the bones of the face, hands, and feet to thicken and enlarge, along with the growth of soft tissues.
Conversely, a deficiency in GH, or hyposecretion, during childhood results in growth failure and a form of proportionate dwarfism.
In adults, GH deficiency is characterized by a set of symptoms including changes in body composition, such as reduced lean body mass and increased fat mass, particularly around the abdomen. Adults with GH deficiency may also report fatigue and a reduction in overall well-being, highlighting the hormone’s continuous role in metabolism and tissue maintenance.