8.2: Hormones

Hormones are classified by their chemical structure, which determines how they travel, bind to receptors, and trigger specific responses in target cells.

Master this section and you'll be able to

• Identify the two major hormone classes by chemical structure and explain how structure affects their solubility, transport, and action on target cells.
• Compare and contrast intracellular and cell membrane hormone receptors.
• Explain how target cell sensitivity is regulated through upregulation and downregulation. 
• Discuss what BPA is and the concerns its use has created.  

Although a given hormone may travel throughout the body in the bloodstream, it will affect the activity only of its target cells; it can only bind to cells that have receptors specifically for that hormone. Once the hormone binds to its specific receptor, a chain of events is initiated that leads to the target cell’s response. Hormones play a critical role in the regulation of physiological processes because of the target cell responses they regulate. These responses contribute to human reproduction, growth and development of body tissues, metabolism, fluid, and electrolyte balance, sleep, and many other body functions.Major Types of Hormones

Hormones of the human body can be divided into two main chemical groups: amino acid–derived hormones (amines, peptides, and proteins) and lipid-derived hormones (steroids). These chemical differences determine how hormones travel in the bloodstream, whether they bind to receptors on the cell surface or inside the cell, and how they influence target cell function.

Figure \(\PageIndex{1}\): Chemical Subdivisions of Hormones.  

 

Amine, Peptide and Protein Hormones

Hormones made from amino acids fall into three main types: amine, peptide, and protein hormones.

• Amine hormones are small molecules made from single modified amino acids, usually tryptophan or tyrosine. For example, melatonin, which is synthesized from tryptophan in the pineal gland, helps regulate the sleep–wake cycle, while the thyroid hormones (T₃ and T₄) and the catecholamines (epinephrine, norepinephrine, etc.), which are derived from tyrosine, help regulate metabolism.

• Peptide hormones are composed of chains of amino acids, typically ranging from 2 to ~ 50 aa in length, though longer chains — called polypeptides — can contain even more. Examples of peptide hormones are insulin and glucagon, both regulating blood sugar levels, working in opposition to each other.

• Protein hormones are longer amino acid chains, with lengths typically exceeding 51 amino acids. One such example is the growth hormone (GH), which stimulates cell growth and repair.

Most amine, peptide, and protein hormones are hydrophilic (water-loving) and therefore water-soluble, allowing them to travel easily through the bloodstream but preventing them from crossing cell membranes. An exception is the thyroid hormones (T3 and T4), which are hydrophobic (lipid-soluble) and can enter target cells directly to influence gene expression.
 

Lipid-Derived (Steroid) Hormones 

The primary hormones derived from lipids are the steroid hormones, which are synthesized from cholesterol. Examples include the reproductive hormones testosterone and estrogens, produced by the gonads (testes and ovaries), as well as aldosterone and cortisol, secreted by the adrenal glands. Aldosterone helps regulate salt and water balance (osmoregulation), while cortisol influences metabolism and the body’s response to stress.

Like cholesterol, steroid hormones are hydrophobic (water-hating), thus fat-soluble and not soluble in water. Because blood is water-based, these lipid-derived hormones must travel through the bloodstream bound to transport proteins. Once they reach their target cells, steroid hormones can easily cross the cell membrane and bind to receptors inside the cell — often in the cytoplasm or nucleus — where they directly influence gene expression and protein synthesis.

In the figure below, you can see seven of the most well-known hormones and the glands where they are produced.

Figure \(\PageIndex{2}\): Some Endocrine Glands and Their Hormones.  The pineal gland (in the brain) releases melatonin, which regulates sleep–wake cycles. The thyroid gland (in the neck) produces thyroxine (T₃ and T₄), which controls metabolism. The pancreas secretes insulin and glucagon to regulate blood glucose levels. The adrenal glands (above the kidneys) release cortisol and epinephrine, important for stress response and metabolism. The ovaries produce estrogen, which regulates female reproductive functions.

Below is the table that lists these important human hormones, their sources, chemical classifications, and main functions in regulating homeostasis.

Hormone Examples

Hormone	Source Gland	Type of Hormone	Main Function
Insulin	Pancreas (β-cells)	Peptide	Lowers blood glucose by promoting cellular uptake of glucose and glycogen formation.
Glucagon	Pancreas (α-cells)	Peptide	Raises blood glucose by stimulating glycogen breakdown and glucose release from the liver.
Cortisol	Adrenal cortex	Steroid	Regulates metabolism and the stress response by increasing blood glucose and reducing inflammation.
Epinephrine (Adrenaline)	Adrenal medulla	Amine (Catecholamine)	Triggers “fight-or-flight” responses: increases heart rate, blood pressure, and energy availability.
Thyroxine (T₄)	Thyroid gland	Amine (Iodinated Tyrosine derivative)	Controls metabolic rate, growth, and development.
Estrogen	Ovaries (and adrenal cortex)	Steroid	Promotes development of female reproductive organs and secondary sexual characteristics.
Melatonin	Pineal gland	Amine (Tryptophan derivative)	Regulates sleep–wake cycles and responds to changes in light and darkness.

 

Pathways of Hormone Action

Hormones deliver their messages by binding to specific receptors, which are proteins located either on the cell membrane or inside the cell. Each receptor is designed to recognize and bind only to hormones with a matching molecular shape, ensuring that signals are highly specific. When a hormone binds to its receptor, it triggers a series of signaling events that lead to the target cell’s response. The same type of receptor may be found in different tissues, but the resulting effects can vary depending on the cell type and its internal machinery.

Once activated, the target cell can respond in several ways. These responses may include stimulating protein synthesis, activating or inhibiting enzymes, changing cell membrane permeability, altering cell growth or division rates, or initiating secretion of cellular products. Importantly, a single hormone can produce different effects in the same cell type depending on which signaling pathways are engaged.

 

The message a hormone sends is received by a hormone receptor, a protein located either inside the cell or within the cell membrane. The receptor will process the message by initiating other signaling events or cellular mechanisms that result in the target cell’s response. Hormone receptors recognize molecules with specific shapes and side groups, and respond only to those hormones that are recognized. The same type of receptor may be located on cells in different body tissues, and trigger somewhat different responses. Thus, the response triggered by a hormone depends not only on the hormone, but also on the target cell.

Once the target cell receives the hormone signal, it can respond in a variety of ways. The response may include the stimulation of protein synthesis, activation or deactivation of enzymes, alteration in the permeability of the cell membrane, altered rates of mitosis and cell growth, and stimulation of the secretion of products. Moreover, a single hormone may be capable of inducing different responses in a given cell.
 

Pathways Involving Intracellular Hormone Receptors

Intracellular receptors (intra = inside; cellular = cell) are located within the cell, either in the cytosol or nucleus. To reach these receptors, a hormone must be able to cross the cell membrane. Steroid hormones, which are derived from cholesterol, can easily diffuse through the lipid bilayer because they are lipid-soluble. Thyroid hormones, which are also lipid-soluble but more complex in structure, enter the cell through specific carrier-mediated transport mechanisms.

Once inside the cell, the hormone binds to its receptor to form a hormone–receptor complex. For steroid hormones, this binding may occur in the cytosol or directly in the nucleus. The complex then attaches to a specific segment of DNA, regulating the transcription of certain genes. In contrast, thyroid hormones bind directly to receptors that are already attached to DNA. In both cases, the interaction between the hormone–receptor complex and DNA initiates gene transcription into messenger RNA (mRNA), which then moves to the cytoplasm and directs protein synthesis by ribosomes, ultimately altering cell function.

Figure \(\PageIndex{3}\): Binding of Lipid-Soluble Hormones.  A steroid hormone directly initiates the production of proteins within a target cell. Steroid hormones easily diffuse through the cell membrane. The hormone binds to its receptor in the cytosol, forming a receptor–hormone complex. The receptor–hormone complex then enters the nucleus and binds to the target gene on the DNA. Transcription of the gene creates a messenger RNA that is translated into the desired protein within the cytoplasm.
 

Pathways Involving Cell Membrane (Extracellular) Hormone Receptors

Hydrophilic (water-soluble) hormones cannot diffuse through the lipid bilayer of the cell membrane, so they must bind to receptors located on the cell’s surface. With the exception of the thyroid hormones, which are lipid-soluble, all amino acid–derived hormones interact with receptors that sit on the extracellular (outer) surface of the membrane.

Because these hormones cannot enter the cell directly, they act as first messengers, triggering a signaling cascade inside the cell through second messengers such as cyclic AMP (cAMP), calcium ions, or other signaling molecules. These second messengers activate specific enzymes and proteins within the cell, ultimately leading to the targeted cellular response — such as changes in metabolism, secretion, or membrane permeability — without directly altering gene transcription.

Figure \(\PageIndex{4}\): Hydrophilic Hormones.  Because hydrophilic (water-soluble) hormones cannot cross the cell membrane, they bind to receptors on the cell surface. When the hormone attaches to its receptor, it triggers a cascade of intracellular signaling events that amplify the message within the cell. These signaling pathways ultimately lead to specific cellular responses, such as activating or inhibiting enzymes, altering membrane permeability, or stimulating protein synthesis and other metabolic processes. (Image credit: "Signal Transduction" by Harmony Folse is licensed under CC BY-NC-SA 4.0, modification of original by Scientific Animations.)

 

Factors Affecting Target Cell Response

For a hormone to trigger a response, the target cell must have specific receptors that recognize and bind to that hormone. However, the strength and duration of the response can vary depending on several factors.

When hormone levels in the bloodstream remain high for an extended period, target cells may decrease the number of available receptors — a process known as downregulation — to reduce their sensitivity and prevent overstimulation. Conversely, when hormone levels are chronically low, cells may increase the number of receptors through upregulation, making them more responsive to the hormone. In addition to changing receptor number, cells can also adjust the sensitivity of existing receptors, fine-tuning their response to maintain balance and effective communication within the endocrine system.

 

Everyday Connection

Bisphenol A (BPA): A Hormone Impersonator in Everyday Life

You have probably seen the phrase “BPA free” on water bottles or food containers — but what exactly is BPA, and why the concern? Bisphenol A (BPA) is a chemical used to make hard plastics and epoxy resins found in many everyday items: food and drink cans, plastic storage containers, water bottles, baby bottles and “sippy” cups,  dental sealants, and even the lining of water pipes. Because these plastics can leach small amounts of BPA into food and liquids, scientists have begun to ask an important question: Is this chemical affecting our hormones?

Research suggests that BPA is an endocrine disruptor — a substance that interferes with the body’s hormonal communication system. BPA can mimic the hormone estrogen and block the actions of androgens (male hormones), which can throw off normal hormone signaling, especially during early development. Studies have linked BPA exposure in fetuses, infants, and young children to possible effects on the brain, behavior, and prostate gland. In adults, high BPA exposure has been associated with thyroid disruption, fertility problems, and changes in metabolism.

While the FDA currently considers low-level exposure to BPA generally safe, ongoing research continues to raise concerns. As a precaution, many U.S. companies have removed BPA from baby bottles, sippy cups, and food packaging. Canada and the European Union have gone a step further, banning BPA entirely from baby products.

To minimize exposure, consumers are advised to:

• Choose “BPA-free” containers and packaging.

• Avoid plastics labeled with recycling codes 3 or 7, which may contain BPA.

• Do not microwave or heat food in plastic containers — use glass, ceramic, or paper instead.

BPA’s story is a powerful reminder that the endocrine system is highly sensitive — even small chemical imbalances can have widespread effects on growth, reproduction, and health.