Understanding the Basics of Endocrinology: A Comprehensive Guide to Hormones and Their Functions

The speaker, Eduardo Paiva, introduces the topic of endocrinology in a series of videos on the physiology of hormones. The series will cover the types of hormones, synthesis, secretion, and hormonal control mechanisms.

The course on endocrinology will be divided into three parts. Part one covers the introduction to endocrinology, types of hormones, synthesis, and secretion. Part two will focus on hormonal transport and control mechanisms. Part three will delve into hormone action and receptor types.

The speaker outlines the specific hormones that will be discussed in the series, including hypothalamic and pituitary hormones, growth hormone, thyroid hormones (T4 and T3), adrenal corticosteroids, insulin, glucagon, parathyroid hormone, calcitonin, and reproductive hormones like testosterone, estrogen, progesterone, luteinizing hormone, and follicle-stimulating hormone.

The course will cover topics related to reproductive physiology, including pregnancy, lactation, ovulation, placenta function, human chorionic gonadotropin, and placental lactogen. Additionally, discussions will include childbirth and lactation processes.

The speaker emphasizes the importance of a systematic and organized approach to studying endocrine physiology. Understanding hormones, their types, functions, and interactions is crucial for a comprehensive comprehension of the subject.

Hormones are chemical messengers produced by endocrine cells that travel through the bloodstream to target cells. They can be classified as endocrine hormones (produced by glands) or neuroendocrine hormones (produced by neurons). Hormones act on specific target cells to regulate various physiological processes.

The study of endocrinology and neuroendocrinology focuses on understanding hormones and their effects. Hormones can be endogenous, autocrine, or paracrine, depending on their secretion and target cells. Other types include pheromones, neuroendocrine hormones, and the nervous system for neuron communication.

The endocrine system, like the nervous system, coordinates communication in the body. It relies on hormones released into the bloodstream to regulate various functions such as metabolism, growth, reproduction, and behavior. Hormones like cortisol, adrenaline, insulin, and thyroid hormones play crucial roles in maintaining bodily functions.

Hormones are classified into three main groups based on their structure: protein/peptide hormones, steroid hormones, and tyrosine-derived hormones. Protein/peptide hormones are the most common type, consisting of amino acids and being hydrophilic. Examples include insulin, growth hormone, and thyroid-stimulating hormone.

Water-soluble hormones are synthesized in cells through a process that involves transcription in the nucleus, translation by ribosomes in the rough endoplasmic reticulum, and packaging in vesicles by the Golgi apparatus. The pre-prohormones are converted into prohormones and then biologically active hormones. These hormones are stored in intracellular fluid until a stimulus triggers their secretion.

Upon stimulation, vesicles containing water-soluble hormones fuse with the cell membrane in a process called exocytosis, leading to successful secretion into the bloodstream. This process ensures the release of biologically active hormones. Water-soluble hormones cannot be stored in lipid vesicles due to their hydrophilic nature.

Insulin serves as an example of the synthesis and secretion of water-soluble hormones. It is produced in the rough endoplasmic reticulum, packaged in vesicles by the Golgi apparatus, and released through exocytosis into the bloodstream. Enzymes in the vesicles facilitate the separation of insulin for secretion.

Steroid hormones, such as cortisol, aldosterone, testosterone, estrogen, progesterone, and 1,25-dihydroxyvitamin D, are liposoluble hormones synthesized from cholesterol. They consist of three cyclohexane rings and one cyclopentane ring. These hormones are secreted by various glands like the adrenal cortex, testes, ovaries, placenta, and kidneys.

Cortisol, a steroid hormone, is synthesized from cholesterol in the mitochondria through intermediaries like pregnenolone. It plays a crucial role in the body's response to stress and is known as the 'stress hormone.' Cortisol is synthesized in the smooth endoplasmic reticulum and easily passes through membranes into circulation.

Hormones derived from the amino acid tyrosine include thyroxine and triiodothyronine secreted by the thyroid gland, and adrenaline and noradrenaline secreted by the adrenal medulla. Thyroid hormones are liposoluble, while adrenal hormones are hydrosoluble.

Thyroid hormones are synthesized by follicular cells in the thyroid gland from iodine stored in a substance called colloid. These liposoluble hormones bind to a transport protein, thyroglobulin, for storage within cells. This mechanism ensures controlled release into circulation.

Peptide hormones and amines are stored in secretory vesicles, while steroids, being liposoluble, do not require storage. Cholesterol esters serve as precursors for steroid synthesis and are stored within cells. Thyroid hormones, although liposoluble, are stored bound to thyroglobulin within cells.

Peptide hormones and amines act extracellularly as they are hydrosoluble, while thyroid hormones, being liposoluble, act intracellularly after release from thyroglobulin. Adrenaline and noradrenaline also act extracellularly due to their hydrosoluble nature.

Hormones are classified based on their structure. They include mostly peptide hormones, derivatives of tyrosine such as adrenaline, noradrenaline, and dopamine, as well as thyroid hormones like thyroxine and triiodothyronine. Additionally, there are steroid hormones. Various glands secrete these hormones, each with specific functions.

Different glands secrete specific hormones. For example, the adrenal cortex releases cortisol and aldosterone, while the adrenal medulla secretes noradrenaline and adrenaline. The pancreas produces insulin and glucagon, the parathyroid gland secretes parathyroid hormone, and the testes and ovaries release testosterone, estrogen, and progesterone. The placenta produces human chorionic gonadotropin, estrogen, and progesterone. The kidneys secrete renin, crucial for the renin-angiotensin-aldosterone system, and erythropoietin for red blood cell production.

Other organs like the heart secrete hormones like atrial natriuretic peptide, the stomach releases gastrin, and the small intestine secretes secretin and cholecystokinin. Adipocytes release leptin, influencing appetite regulation. Understanding the diverse functions of hormones is essential in the comprehensive study of endocrinology.

Future discussions in endocrinology will delve into hormonal transport, regulation, and feedback mechanisms. The focus will be on both positive and negative feedback loops, as well as hormone clearance processes. These topics aim to provide a holistic understanding of the endocrine system.

The video is dedicated to Dr. Wagner Pradela, a close friend and endocrinology enthusiast. Acknowledgments are extended to mentors like Dr. Osvaldo Santesteban, Dr. Mauricio Don, and Dr. Escolar Migueles Howard Brown for their valuable teachings. The speaker expresses gratitude to colleagues and teachers, citing the use of Gaitán's Physiology Treatise in the video.