Hormones
Published: Jul 17, 2023
/
Updated: Jul 27, 2023
Written by Oseh Mathias
Founder, SpeechFit
Hormones are chemical messengers that are produced and released by endocrine glands or specialised cells in various organs throughout the body [1].
The production and release of hormones are regulated by complex feedback mechanisms within the body. Endocrine glands, such as the pituitary gland, thyroid gland, adrenal glands, and others, synthesize and release hormones directly into the bloodstream [2]. These hormones then travel throughout the body, reaching their target tissues or organs, where they bind to specific receptors and initiate specific physiological responses [3].
Hormones are synthesized in different ways depending on their chemical structure. They can broadly be classified into three groups: peptides/proteins, steroids, and amines [5].
Peptide and protein hormones are composed of chains of amino acids, while steroid hormones are derived from cholesterol, and amine hormones are derived from the amino acid tyrosine [6].
Peptide and Protein Hormones: These are synthesised as larger precursor molecules in the rough endoplasmic reticulum of the endocrine cells. The precursor molecules are then processed in the ER and Golgi apparatus to yield the active hormone, which is stored in vesicles within the cell until it's released in response to specific signals. Insulin and growth hormone are examples of peptide hormones [7].
Steroid Hormones: These are derived from cholesterol and are synthesised in the smooth endoplasmic reticulum and mitochondria of endocrine cells. Unlike peptide hormones, steroid hormones aren't stored in vesicles; instead, they are released immediately into the bloodstream following synthesis. Cortisol, testosterone, and estradiol are examples of steroid hormones [8].
Amine Hormones: These are derived from the amino acids tyrosine and tryptophan. The process of their synthesis varies but generally involves enzymatic modification of the amino acids within endocrine cells. Examples include thyroxine (produced by the thyroid gland) and the catecholamines epinephrine and norepinephrine (produced by the adrenal glands) [9].
Hormones exert their effects by binding to receptors on or inside their target cells. The type of receptor a hormone binds to depends on the chemical nature of the hormone[11].
Peptide and Amine Hormones: These are water-soluble and cannot cross the lipid bilayer of the cell membrane. They bind to receptors on the surface of their target cells. These receptors are often part of a larger complex that includes a G-protein and an effector enzyme[12]. Binding of the hormone activates the G-protein, which then activates the effector enzyme, leading to the generation of a second messenger (like cyclic AMP) inside the cell[13]. The second messenger then triggers a series of events that leads to the cell's response to the hormone[14].
Steroid Hormones: These are lipid-soluble and can cross the cell membrane to bind to intracellular receptors (found in the cytoplasm or nucleus)[15]. The hormone-receptor complex then moves to the nucleus, where it acts as a transcription factor to regulate the expression of specific genes, leading to changes in the cell's activity[16].
Thus, hormones work in precise ways, binding to specific receptors like a lock and key, to regulate the functions of different cells and organs[17].
Common Hormones
| Hormone | Source | Function |
|---|---|---|
| Insulin | Pancreas | Regulates blood glucose levels by promoting its uptake by cells[18] |
| Glucagon | Pancreas | Increases blood glucose levels by promoting its release from storage[18] |
| Growth Hormone (GH) | Pituitary gland | Stimulates growth and development, regulates metabolism[19] |
| Thyroid Stimulating Hormone (TSH) | Pituitary gland | Stimulates the production and release of thyroid hormones[20] |
| Thyroxine (T4) | Thyroid gland | Regulates metabolism, growth, and development[21] |
| Triiodothyronine (T3) | Thyroid gland | Regulates metabolism, growth, and development[21] |
| Adrenaline (Epinephrine) | Adrenal glands | Prepares the body for "fight-or-flight" response in stress situations[22] |
| Cortisol | Adrenal glands | Regulates stress response, metabolism, and immune system[23] |
| Testosterone | Testes (males), Ovaries (females) | Promotes male sexual characteristics, regulates libido in both sexes[24] |
| Estrogen | Ovaries (females) | Promotes female sexual characteristics, regulates menstrual cycle[25] |
| Progesterone | Ovaries (females) | Prepares the uterus for pregnancy, regulates menstrual cycle[26] |
| Oxytocin | Pituitary gland | Facilitates childbirth, stimulates lactation, promotes bonding[27] |
| Prolactin | Pituitary gland | Stimulates milk production in lactating women[28] |
| Melatonin | Pineal gland | Regulates sleep-wake cycles and circadian rhythms[29] |
| Serotonin | Various glands and neurons | Regulates mood, appetite, and sleep[30] |
| Follicle-Stimulating Hormone (FSH) | Pituitary gland | Stimulates the development of eggs in females and sperm in males[31] |
| Luteinizing Hormone (LH) | Pituitary gland | Triggers ovulation in females, stimulates testosterone production in males[32] |
Hormones circulate throughout the body in the bloodstream, interacting with specific target cells and organs to induce certain physiological responses[33].
Some of their primary functions can be found below:
Regulation of Metabolism: Hormones like insulin and glucagon, produced by the pancreas, help regulate glucose levels in the blood[34]. Thyroid hormones, thyroxine (T4) and triiodothyronine (T3), control the metabolic rate of the body[35].
Growth and Development: Growth hormone (GH), produced by the pituitary gland, promotes growth in children and helps maintain healthy body composition in adults[36]. Thyroid hormones also play a crucial role in the development of the brain and overall growth[37].
Reproduction: Hormones such as estrogen, progesterone, and testosterone play key roles in the reproductive systems of males and females[38]. They regulate menstrual cycles, sperm production, and are also essential for sexual development and function[39].
Response to Stress and Injury: Hormones like cortisol, epinephrine (adrenaline), and norepinephrine help the body respond to stress or injury[40]. They prepare the body for "fight or flight" in response to perceived threats and help manage inflammation and tissue repair[41].
Maintenance of Homeostasis: Hormones are crucial for maintaining homeostasis in the body, which is the state of steady internal physical and chemical conditions[42]. They do this by balancing fluid and electrolyte levels, adjusting metabolism, and controlling body temperature, among other things[43].
Regulation of Sleep-Wake Cycle: Melatonin, a hormone produced by the pineal gland, helps regulate the sleep-wake cycle[44].
Mood and Behaviour: Hormones can also influence mood and behavior. For example, serotonin and dopamine, often classified as neurotransmitters, function like hormones outside of the brain and play crucial roles in mood regulation[45].
While both hormones and neurotransmitters are chemical messengers that regulate various physiological processes, there are some key differences between them:
Site of Release: Hormones are released into the bloodstream by endocrine glands or specialised cells, allowing them to reach distant target tissues or organs throughout the body[46]. Neurotransmitters are released locally within the nervous system and act over short distances within the synaptic cleft[47].
Mode of Transport: Hormones travel through the bloodstream to reach their target tissues. In contrast, neurotransmitters travel across synapses in a rapid and localised manner[48].
Target Cells: Hormones generally act on specific target cells or organs, often affecting multiple cells simultaneously[49]. Neurotransmitters, on the other hand, act on specific receptors on individual neurons or, in some cases, on adjacent cells, such as muscle cells or glandular cells[50].
Time Course: Hormonal responses tend to have a slower onset and longer duration, as they require circulation throughout the body[51]. Neurotransmitter responses are rapid and have a shorter duration, enabling fast communication within the nervous system[52].
There can be some overlap, as some substances can act as both neurotransmitters and hormones, depending on their site of release and mode of action. For example, epinephrine (adrenaline) functions as both a hormone, released by the adrenal glands into the bloodstream during stress responses, and a neurotransmitter, released within the nervous system in response to certain stimuli[53].
Oseh is a software engineer, entrepreneur and founder of SpeechFit. Oseh is passionate about improving health and wellbeing outcomes for neurodiverse people and healthcare providers alike.
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