Types of Hormones

Hormone Functions

The endocrine system plays a role in growth, metabolism, and other processes by releasing hormones into the blood.

Learning Objectives

Evaluate hormones and their purpose in the body

Key Takeaways

Key Points

  • Hormones serve as chemical messengers in the body and help maintain homeostasis.
  • Hormones are released into bodily fluids, like blood, which carry them to target cells.
  • Target cells respond to a hormone when they express a specific receptor for that hormone.
  • Hormones also play a role in the regulation of cell death, the immune system, reproductive development, mood swings, and hunger cravings.
  • In the adrenal gland, epinephrine and norepinephrine regulate responses to stress; in the thyroid gland, thyroid hormones regulates metabolic rates.

Key Terms

  • target cell: any cell having a specific receptor for a hormone
  • hormone: any substance produced by one tissue and conveyed by the bloodstream to another to affect physiological activity
  • endocrine system: a control system of ductless glands that secrete hormones which circulate via the bloodstream to affect cells within specific organs

The Endocrine System: Hormones

An animal’s endocrine system controls body processes through the production, secretion, and regulation of hormones. Hormones serve as chemical “messengers” that function in cellular and organ activity to maintain the body’s homeostasis. Maintaining homeostasis within the body requires the coordination of many different systems and organs. Communication between neighboring cells and between cells and tissues in distant parts of the body occurs through the release of hormones into body fluids (usually blood), which carry them to their target cells. Target cells, those having a receptor for a signal, respond to a hormone when they express a specific receptor for that hormone. Cellular recipients of a particular hormonal signal may be one of several cell types that reside within a number of different tissues, as is the case for insulin, which triggers a diverse range of systemic physiological effects. Different tissue types may also respond differently to the same hormonal signal.

By releasing hormones, the endocrine system plays a role in growth, metabolism, and sexual development. Hormones also play a role in induction or suppression of cell death, activation or inhibition of the immune system, mood swings, and hunger cravings. In humans, common endocrine system diseases include thyroid disease and diabetes mellitus.

Examples of endocrine glands include the adrenal glands, which produce hormones, such as epinephrine and norepinephrine that regulate responses to stress, and the thyroid gland, which produces thyroid hormones that regulate metabolic rates. In organisms that undergo metamorphosis, the process is controlled by the endocrine system. The transformation from tadpole to frog, for example, is complex and nuanced to adapt to specific environments and ecological circumstances.

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Hormones in metamorphosis: The process of amphibian metamorphosis, as seen in the tadpole-to-frog stages shown here, is driven by hormones.

Lipid-Derived, Amino Acid-Derived, and Peptide Hormones

All hormones in the human body can be divided into lipid-derived, amino acid-derived, and peptide hormones.

Learning Objectives

Recognize characteristics associated with lipid-derived, amino acid-derived, and peptide hormones

Key Takeaways

Key Points

  • Most lipid hormones are steroid hormones, which are usually ketones or alcohols and are insoluble in water.
  • Steroid hormones (ending in ‘-ol’ or ‘-one’) include estradiol, testosterone, aldosterone, and cortisol.
  • The amino acid – derived hormones (ending in ‘-ine’) are derived from tyrosine and tryptophan and include epinephrine and norepinephrine (produced by the adrenal medulla).
  • Amino acid-derived hormones also include thyroxine (produced by the thryoid gland) and melatonin (produced by the pineal gland).
  • Peptide hormones consist of a polypeptide chain; they include molecules such as oxytocin (short polypeptide chain) or growth hormones ( proteins ).
  • Amino acid-derived hormones and protein hormones are water-soluble and insoluble in lipids.

Key Terms

  • oxytocin: a hormone that stimulates contractions during labor, and then the production of milk
  • epinephrine: (adrenaline) an amino acid-derived hormone secreted by the adrenal gland in response to stress
  • estrogen: any of a group of steroids (lipid-hormones) that are secreted by the ovaries and function as female sex hormones

Types of Hormones

Although there are many different hormones in the human body, they can be divided into three classes based on their chemical structure: lipid-derived, amino acid-derived, and peptide hormones (which includes peptides and proteins). One of the key, distinguishing features of lipid-derived hormones is that they can diffuse across plasma membranes whereas the amino acid-derived and peptide hormones cannot.

Lipid-Derived Hormones (or Lipid-soluble Hormones)

Most lipid hormones are derived from cholesterol, so they are structurally similar to it. The primary class of lipid hormones in humans is the steroid hormones. Chemically, these hormones are usually ketones or alcohols; their chemical names will end in “-ol” for alcohols or “-one” for ketones. Examples of steroid hormones include estradiol, which is an estrogen, or female sex hormone, and testosterone, which is an androgen, or male sex hormone. These two hormones are released by the female and male reproductive organs, respectively. Other steroid hormones include aldosterone and cortisol, which are released by the adrenal glands along with some other types of androgens. Steroid hormones are insoluble in water; they are carried by transport proteins in blood. As a result, they remain in circulation longer than peptide hormones. For example, cortisol has a half-life of 60 to 90 minutes, whereas epinephrine, an amino acid derived-hormone, has a half-life of approximately one minute.

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Lipid-derived hormones: The structures shown here represent (a) cholesterol, plus the steroid hormones (b) testosterone and (c) estradiol.

Amino Acid-Derived Hormones

The amino acid-derived hormones are relatively small molecules derived from the amino acids tyrosine and tryptophan. If a hormone is amino acid-derived, its chemical name will end in “-ine”. Examples of amino acid-derived hormones include epinephrine and norepinephrine, which are synthesized in the medulla of the adrenal glands, and thyroxine, which is produced by the thyroid gland. The pineal gland in the brain makes and secretes melatonin, which regulates sleep cycles.

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Amino acid-derived hormones: (a) The hormone epinephrine, which triggers the fight-or-flight response, is derived from the amino acid tyrosine. (b) The hormone melatonin, which regulates circadian rhythms, is derived from the amino acid tryptophan.

Peptide Hormones

The structure of peptide hormones is that of a polypeptide chain (chain of amino acids). The peptide hormones include molecules that are short polypeptide chains, such as antidiuretic hormone and oxytocin produced in the brain and released into the blood in the posterior pituitary gland. This class also includes small proteins, such as growth hormones produced by the pituitary, and large glycoproteins, such as follicle-stimulating hormone produced by the pituitary.

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Peptide hormones: The structures of peptide hormones (a) oxytocin, (b) growth hormone, and (c) follicle-stimulating hormone are shown. These peptide hormones are much larger than those derived from cholesterol or amino acids.

Secreted peptides, such as insulin, are stored within vesicles in the cells which synthesize them. They are then released in response to stimuli (e.g., as high blood glucose levels in the case of insulin). Amino acid-derived and polypeptide hormones are water-soluble and insoluble in lipids. These hormones cannot pass through plasma membranes of cells; therefore, their receptors are found on the surface of the target cells.