Hormones communicate between organs.
An endocrine hormone is a molecule produced by a hormone-secreting gland and carried by the blood to another organ, whose behavior it controls. Generally, the Hormone plugs into a specific receptor protein in the target cell, changing the shape of the receptor protein and thereby, producing a change in cell function.
Hormones are used to communicate between organs and tissues to regulate physiological and behavioral activities, such as digestion, metabolism, respiration, tissue function, sleep, excretion, lactation, stress, growth and development, movement, reproduction and mood.
It is also possible for a cell to produce a hormone to control activity within itself [an “Intracrine” hormone] or to control nearby cells in the same organ [a “paracrine” hormone].
There are many kinds of hormone molecules: eicosanoids, steroids, amino acid derivatives, peptides, proteins etc.
Some are soluble in water, while some only dissolve in oils and fats (lipids).
The lipid-soluble ones need special transport proteins in the blood, to transport them from their gland of origin, to the target organ.
Hormones pack a powerful punch, in terms of their effect on the body. Therefore they need to be “balanced”: produced in exactly the right amounts, at the right time, to keep the target organs running smoothly and to keep the body as a whole, “in tune”. This is achieved by a delicate; but effective “feedback system” in which a chemical [often, another hormone] produced by the target cell goes back to the hormone producing gland, which can tell how much target hormone is present in the blood. The secreting gland then fine-tunes its hormone production accordingly.
Hormones are also produced in a “diurnal rhythm”, with maximum production at a particular time of day. Each hormone has its own rhythm. For example thyroid 3 hormone is boosted at 4 AM and testosterone at 8 AM.
The body is a bit like an orchestra, with the brain, the Hypothalamus and the Pituitary gland controlling the function of the peripheal glands and organs: all glands and every organ is supervised and instructed by adjusting the levels of various controlling hormones.
 The Pineal gland is the (diurnal) timer, to tell the system when to sleep and when to play.
 The Hypothalamus detects input signals from the hormonal glands, skin nerves, blood chemicals, intestines, eyes, ears and emotions (in fact, the whole body), and balances the output of control hormones by the Pituitary.
 The Pituitary gland, like a conductor, tells the individual players when and how forcefully, to play their part.
If all is in balance, the individual player does just the right amount of work and the system is in tune.
Outside of the brain, there are many players:
Major “generalist” glands like the skin, the thyroid, the parathyroids, the pancreas, the intestine, the adrenals and the gonads (ovaries in the female and testicles in the male) produce hormones which affect the entire body,
“Specialist” hormone producers like the kidneys, stomach, bowel, placenta and the fat make hormones which are included in the Hypothalamus’ calculations, but also act directly on other specific glands or tissues.
Micro factories in all the cells process Pregnenolone, DHEA, Levothyroxine (“thyroid #4 hormone”, or “T4”), Cortisol and other “generalist” hormones into specific molecules for that cell’s own maintenance and repair.
WHY DO WE NEED THEM?
Every cell in your body, from your skin and hair, to your brain and all your other parts, dances to the beat of your hormones and can only function optimally when they are “in tune”.
Hormonal imbalance can be responsible for physical dysfunction, including obesity, for cognitive dysfunction including “fuzzy thinking”, for psychological dysfunction, including anxiety, panic attacks and depression and perhaps for Alzheimer’s and other neurological diseases.
Therefore in a significant proportion of cases, hormonal “restoration” or “rebalancing” will return the individual to stable, normal function.
G. A. Harry, MD.
REFERENCES, FOR “WHAT IS A HORMONE AND WHY DO WE NEED THEM?”:
1: DEFINITION – Wikipedia: https://en.wikipedia.org/wiki/Hormone
2: RECEPTORS – Wikipedia: https://en.wikipedia.org/wiki/Receptor_(biochemistry)
3: TRANSPORT PROTEINS – Wikipedia: https://en.wikipedia.org/wiki/Transport_protein
4: PINEAL GLAND – Britannica: https://www.britannica.com/science/pineal-gland
5: HYPOTHALAMUS – Britannica: https://www.britannica.com/science/hypothalamus
[A]: 17 BETA HYDROXY STEROID DEHYDROGENASES https://www.ncbi.nlm.nih.gov/pubmed/11091120
[B] INTRACRINOLOGY (Prof Fernand Labrie) https://www.ncbi.nlm.nih.gov/pubmed/10915214
8: THYROID GLAND FUNCTION – https://mcb.berkeley.edu/courses/mcb135e/thyroid.html
9: REGARDING AGING – https://www.nature.com/articles/d41586-018-05582-3
10: EVERYTHING YOU WANTED TO KNOW ABOUT THE PINEAL GLAND – https://www.globalhealingcenter.com/natural-health/everything-you-wanted-to-know-about-the-pineal-gland/#1
G. A. Harry, MD