Hormones of the Body

Regulation of Release

Hormone release is controlled by three general mechanisms:

Regardless of the mechanism of control, hormone release is typically regulated through negative feedback loops. For example, if blood glucose levels increase, insulin secretion will increase which will lower the levels of glucose in the blood. As glucose levels return toward normal insulin secretion will decrease.

Negative Feedback Loop using example Glucocorticoids. When hormone levels become elevated, a negative signal is sent to the pituitary gland and hypothalamus to turn down the release of further hormone
Author: Open Stax; License: [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons  Link: https://cnx.org/resources/531ca03c11f2ff25b845d1dfd86993c058f3424c/1805_Negative_Feedback_Loop.jpg

Transport: Endocrine hormones are transported to their target tissues via the blood plasma. Since the plasma is mostly water this presents a problem for some hormones. Hydrophilic hormones easily dissolve in the plasma and pose no major problem for transport. Hydrophobic hormones, on the other hand, cannot dissolve in the plasma and so this presents a challenge for their transport. These hormones must bind to plasma proteins produced by the liver in order to be transported. These "carrier" proteins have hydrophobic cores that shield the hormones from the aqueous plasma. However, even for these hydrophobic hormones a small fraction does circulate free in the plasma. We distinguish between the two forms as "bound" and "free" hormone. It is the ratio of bound to free that determines the overall potency of the hormone. The free hormone is the biologically active form and as the levels of free hormone drop, more bound hormone will be released from the transport proteins to maintain the equilibrium ratio of bound to free.

Life span of a hormone: The only way to turn off an endocrine response is to remove the hormone from the circulation. There are several mechanisms for removing the hormones. These include: removal by the kidneys, removal by the liver, enzymatic destruction of the hormone, and re-uptake and recycling of the hormone. Steroid hormones, for example, are usually removed from the blood by the liver while protein hormones often end up in the urine. We express the life span of a hormone in the blood as its half-life (T ½). Recall that one half-life is the amount of time necessary to remove 50% of the hormone from the circulation. Half-lives for hormones range from a few minutes to several days. The concentration in the blood of hormones with short half-lives tend to fluctuate markedly while the concentrations of hormones with longer half-lives tend to be more constant. Since hormones bound to carrier molecules are shielded from the mechanisms that remove them from the blood their half-lives tend to be longer and their concentrations do not fluctuate as rapidly. 

The concentrations of hormones in the blood are typically very low, ranging from 10-11 to 10-9 moles/Liter. Thus, hormone receptors must have very high affinities for their particular hormone. The affinity of a hormone for its receptor is a measure of how easily and strongly it binds to the receptor. Hormone affinities are expressed as the dissociation constant (KD) for the hormone. The units of the dissociation constants are molar units (M) and correspond to the concentration of the hormone required to bind exactly one-half of available receptors. The lower the KD number the higher the affinity of the receptor to the hormone.

There are many different hormones in the body, and making an accurate count is nearly impossible as new hormones are discovered every year. However, there are general groupings that can help distinguish the characteristics of the vast arrays of hormones. One common method of classifying hormones is based on their chemical structure, this type of classification results in three main classes of hormones: peptide/protein hormones, steroid hormones, and amino-acid derived hormones. The hormones within each class have similar functional properties. The table below describes the characteristics of each class of hormones based on the 5 properties define below.

  1. Synthesis: How is the hormone synthesized?  It may be produced on demand or stored for later release.
  2. Mode of release: Is the hormone released from vesicles through exocytosis or simply produced and allowed to diffuse out of the cell.
  3. Transport: How is the hormone transported in the blood? It may circulate free or it may be bound to carrier proteins.
  4. Half-life: How long does the hormone circulate in the blood before being broken down. It may be broken down quickly or it may stay in the circulation for hours or even days. 
  5. Receptor: Which kind of receptors does it interact with (See Table below)

Classes of Hormones

 Peptide/ProteinSteroidAmino-Acid Derived
SynthesisProduced as inactive forms and stored in vesiclesMade on demand from cholesterolProduced and stored in vesiclesProduced and stored as precursor
ReleaseExocytosisDiffusionExocytosisFacilitated diffusion using a carrier
TransportDissolved in plasma (water-soluble)Bound to carrier proteins (lipid-soluble)Dissolved in plasma (water-soluble)Bound to carrier proteins (lipid-soluble)
Circulatory half-lifeMinutes100s of minutesLess than a minuteDays
Receptor typeMembrane bound receptorsCytosolic or nuclear receptorsMembrane bound receptorsNuclear receptors
ExamplesInsulin, Growth HormoneEstrogen, TestosteroneEpinephrine, NorepinephrineThyroxine

As the name suggests, peptide/protein hormones are produced from amino acids and range in size from three amino acids to hundreds of amino acids. This is the most diverse and abundant type of hormone group and is made by tissues located throughout the body. Steroid hormones are always derived from cholesterol (and therefore hydrophobic) and are made by only a few organs, specifically the gonads and adrenal cortex. Hormones categorized as amino-acid derived are created from the amino acid tyrosine (catecholamines and thyroid hormones) or tryptophan (melatonin and serotonin). The tables below list the major endocrine glands, the hormones they produce and the major actions of the hormones.

Major Endocrine Glands and the Hormones They Secrete

HormoneTarget TissuePrimary Action
Gonadotropin-Releasing Hormone (GnRH)Anterior PituitaryStimulate secretion of Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH)
Thyrotropin-Releasing Hormone (TRH)Anterior PituitaryStimulate secretion of Thyroid Stimulating Hormone (TSH)
Corticotropin-Releasing Hormone (CRH)Anterior PituitaryStimulate secretion of Adrenocorticotrophic Hormone (ACTH)
Growth Hormone-Releasing Hormone (GHRH)Anterior PituitaryStimulate secretion of Growth Hormone (GH)
Growth Hormone-Inhibiting Hormone (GHIH, somatostatin)Anterior PituitaryInhibit secretion of GH
Prolactin-Inhibiting Hormone (PIF, dopamine)Anterior PituitaryInhibit secretion of Prolactin (PRL)
Anterior Pituitary
HormoneTarget TissuePrimary Action
Thyroid-stimulating Hormone (TSH)Thyroid GlandStimulate secretion of Thyroxin (T4) and Triiodothyronine (T3)
Follicle-stimulating Hormone (FSH)Ovaries and TestesMale: Sperm production Female: Follicle development and Estrogen secretion
Luteinizing Hormone (LH)Ovaries and TestesMale: Testosterone production Female: Ovulation, Progesterone secretion
Adrenocorticotrophic Hormone (ACTH)Adrenal CortexStimulate secretion of Glucocorticoids (Cortisol)
Growth Hormone (GH)Most tissuesStimulates tissue growth Regulation of metabolism
ProlactinMammary glands Stimulates milk production Up-regulation of FSH and LH receptors
Posterior Pituitary
HormoneTarget TissuePrimary Action
OxytocinUterus and mammary glandsStimulates uterine contractions Stimulates release of milk Social and moral feelings (Brain)
Antidiuretic Hormone (ADH) (Vasopressin)Kidneys and blood vesselsRenal water reabsorption (reduced urine volume) Vasoconstriction
Thyroid gland
HormoneTarget TissuePrimary Action
Thyroxine (T4)Whole BodyMetabolism and Growth
Triiodothyronine (T3)Whole BodyMetabolism and Growth
Parathyroid glands
HormoneTarget TissuePrimary Action
Parathyroid Hormone (PTH)BoneIncrease blood calcium
HormoneTarget TissuePrimary Action
InsulinSkeletal muscle, Adipose tissue, LiverLowers blood glucose levels
GlucagonLiverRaises blood sugar levels by stimulating glycogen breakdown and glucose synthesis
Adrenal glands
HormoneTarget TissuePrimary Action
Adrenal Cortex: Mineralocorticoids (Aldosterone)KidneyIncreased Na+ reabsorption and Excretion, increased water reabsorption
Adrenal Cortex: Glucocorticoids (Cortisol)Most tissuesIncreased protein and lipid breakdown Increased glucose production (increased blood sugar) Anti-inflammatory
Adrenal Cortex: AndrogensMany tissuesNot as important in males In females stimulates growth of axillary and pubic hair
Adrenal Medulla: Epinephrine and NorepinephrineMany tissuesIncrease blood glucose (glycogen breakdown) Fight-or-flight response
HormoneTarget TissuePrimary Action
Testosterone (Male)Most tissuesMale sexual development Spermatogenesis
Estrogen (Female)Most tissuesFemale sexual development
Progesterone (Female)Many tissuesGestation Maternal behavior
Digestive Tract
HormoneTarget TissuePrimary Action
GastrinParietal CellsGastric acid secretion
Cholecystokinin (CCK)Gall bladder, Pancreas, StomachRelease of bile from gall bladder Secretion of digestive enzymes by pancreas Decreased stomach emptying
SecretinPancreas, LiverIncreased bicarbonate secretion by pancreas and liver
Gastric Inhibitory Peptide (GIP)Beta cells of pancreasIncreased insulin secretion

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