In 17th century England, one of the earliest and most important steps towards the exploration of human physiology began, when Thomas Wharton sought to deny a commonly held belief about the nature of the human brain-up until that time, the brain was thought to be a gland responsible for the secretion of mucus. The difference between ductless and ductile glands was also initially illustrated and explained by Wharton, who after careful examination made a clear distinction in the different functions concerning the endocrine and the exocrine system. The existence of hormones was initially elucidated in the 17th century, when the first observations concerning their existence began to filter into the scientific community. The respected Dutch scientist, Fredrik Ruysch, in the 1690s, made a claim about the nature and function of the thyroid gland; he suggested that this gland poured important substances into the blood stream. the Frenchman, Theophile Bordeu, thirty years earlier had made a remarkable declaration, which implied that some parts of the human body gave off certain "emanations" which seem to induce very dramatic effects on all the other parts of the body - for this work, he is still regarded as the founder of modern endocrinology by some scientists. As knowledge about endocrine glands progressed towards the end of the 18th century, many doctors started to associate the swelling of the neck, excessive staring, or the "bug" eyes, the presence of a rapid and racing pulse, and the presence of uncontrollable muscular tics or tremors with the swelling in the thyroid gland. Goiter was recognized, in this disorder, patients begin to develop so much distension in the gland that it can induce a huge and disfiguring growth on the neck, as if the bulging mass was attempting to burst out from the front of the patient's neck. More correctly, such an enlargement of the thyroid is now known as toxic goiter, the production of excessive amounts of the thyroid hormone is responsible for this particular disorder. The study of diseased glands from different patients was the source for the earliest information about the functioning of the human endocrine system. The British scientist Thomas Addison studied eleven people with the symptoms of anemia in 1849, he found that their blood did not contain sufficient amount of a chemical compound known as hemoglobin, this is the compound which forms the active center of the human red blood cells, it is responsible for the transport of oxygen to different parts of the body by binding with the gas. The patients that Addison studied often felt faint or were lethargic, he also noticed that all of them had very weakened cardiac functioning; the other physical symptoms he noticed was that the skin of the patients had a very sickly or gray coloration. Addison examined the symptoms and the patients themselves died soon. He went one step further and cut the dead men open in the form of an early and experimental autopsy, making the discovery that all of the patients had diseased adrenal glands - he came to the conclusion that the disease in the adrenal glands was what prompted the symptoms and caused death. The name "melasma suprarenale" was given to the condition by Addison; we now call this disorder of the adrenal glands as Addison's disease. A German doctor, A. A. Berthold, also released the results of his experiments carried out on six young male chickens the same year that Addison discovered the adrenal disorder. The experimental results with the chickens were the first experimental proof which confirmed that hormones existed and it also shed some light on the nature of their function. The experiment conducted by Berthold, involved the castration of four of the male chickens. While the remaining two chickens were left of with their testes whole, the scientist also went further, and transplanted the removed testes of two of the chickens back into their bodies, however, these were now placed at a distant location and far off from where the tested normally resided in the body. The growth of two unmolested chickens was normal and they became roosters in time, they sprouted normal combs and wattles, and had the plumage of normal adult roosters. In comparison, two chickens that underwent castration without corresponding transplant of the testes back into the body never developed any of the adult male characters, in these two chickens there was a complete atrophy or shrinkage of the comb. The result gave the first solid proof and connection between hormonal deficiency and physiological growth. What spiked interest and provided a glimpse into the nature of hormones further, were the results seen in the two castrated birds which had transplanted or relocated testes - in this case, these two birds also developed into very normal looking and sexually mature roosters. This singular experiment was the first experimental evidence which suggested to scientist that hormones travel freely via the bloodstream. Thus it became known that as far as some hormones were concerned, the place of origination is not necessarily the crucial factor in the functioning - this result suggested that even displaced glands would be efficient as long as a direct connection to the circulatory system was available.
Some sort of disorder with the endocrine system strikes about ten percent of the people in any population of the majority of developed nations. A glandular problem is involved in the majority of all endocrine system disorders, these can include the presence of a tumor. Endocrine or glandular disorder can induce either overproduction of the hormone - hyper functioning of the secretory organ or they can cause an underproduction of hormone - hypo functioning of secretory organ, both extremes result in imbalances which induce all sorts of unpleasant to severe disorders. Problems such as hyperthyroidism and hypothyroidism in the thyroid are typical endocrine disorders. The presence of these conditions can induce enlargement of the gland or the formation of additional tumors in the glandular tissue. A very visible symptom of toxic goiters is a mark of hyperthyroidism - namely the excess production go the hormone thyroxin in the thyroid gland. On the other hand, the converse condition of hypothyroidism induces simple goiter, this is a common condition in many underdeveloped parts of the world and comes about primarily because of diets that lack the essential mineral iodine. The occurrence of this condition in developed countries is very rare today as diets are generally excellent and because iodine is almost always a common ingredient in all table salt products. Endocrine disorders are not always caused by the overproduction or the underproduction of glandular hormones, they can arise due to other factors such as those that involve the enhancement in sensitivity or the diminishment in the sensitivity of target cells receptors. A malfunctioning, defective or absent receptor cannot receive hormones and these cause the deaths or some disabilities to their possessors - they can be due to biochemical or hereditary factors or even toxic factors. Furthermore, many genetic defects which cause or give rise to abnormalities in the hormone synthesis can also cause disorders similar in symptomatic attributes to endocrine disorders. Endocrine disorders can also arise due to the presence of tumors and cysts, or the presence of infections in the endocrine glands. The simple administration of relevant hormones is usually the normal way to treat underproduction, or hypo function. These hormones are usually administered directly into the bloodstream of the patient to correct the deficiency. The use of a totally different hormone or even a separate chemical compound may substitute the necessary hormone in the treatment of some cases. Hyper function of endocrine glands or the treatment of overproduction of hormones can take a variety of different routes. Tumors causing a gland to hyper function, are usually removed for simple and effective treatment. The use of drugs which can block the production of specific hormones or block receptors for these hormones in the target tissues is another way to treat such disorders.