Hormones are chemical messengers produced by the endocrine system that serve as chemical messengers throughout the body. Hormones regulate the body's processes, like hunger, reproduction, and sexual desire, by communicating with their target cell(s).

What are hormones?

Hormones are the body’s chemical messengers, travelling through the bloodstream to tissues and organs to transmit various signals. Produced by the endocrine system, hormones play a number of crucial roles in the body, and have a direct impact on development and growth, metabolism, sexual function, and cognitive abilities.

Hormones are secreted directly into the body through endocrine glands, such as the pituitary gland, the pineal gland, and the thymus gland (among others). Reproductive hormones include testosterone, estrogen, androgen, and progesterone. In males, these hormones are produced in the testes, while in females they develop in the ovaries. Reproduction in all sexes also involves follicle-stimulating hormone (FSH) and luteinizing hormone (LH), both of which are produced in the pituitary gland.

Hormones and male fertility

It’s uncommon, but possible, for male-factor infertility to be caused by a hormonal deficiency.  LH and FSH are responsible for producing sperm and testosterone in the testes. Low testosterone levels, also known as “hypogonadism”  does not usually cause infertility, but can lead to a reduced sex drive or problems sustaining an erection. Patients with lower levels of testosterone can typically still conceive, but imbalances in FSH or LH can affect sperm production. 

History of hormones

German physiologist and zoologist Arnold Adolph Berthold first discovered testosterone in 1849, following an experiment with roosters. He observed that, without testes, male chickens did not develop male characteristics. But when the testes were replaced, the location did not matter — rather, he concluded, there must be a chemical secreted by the testes that controls this development.

In 1902, William Bayliss and Ernest Starling made significant contributions of their own in the discovery of hormones when they were studying the nervous system’s impact on the digestive system. The duo concluded that there was a factor secreted from the bloodstream to the intestines that regulated the digestive tract. They named it secretin. It was only until 1905, three years later, that the term “hormones” was coined by Starling.

Finally, the work of Charles Darwin and his son Francis — who studied the movements of plants towards light, — eventually led to the discovery of the first plant hormone in the 1920s by Frits Warmolt Went. 

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