Endocrine System Overview

The endocrine system is composed of glands without ducts, which secrete hormones directly into the bloodstream via blood sinusoids or fenestrated capillaries. These glands include the pituitary, adrenal, thyroid, parathyroid, and pineal.

Hormones are chemical substances secreted by specific cells that affect distant organs. They regulate biochemical reactions and bodily processes like growth, maturation, regeneration, and reproduction.

Hormones are classified into three main types: protein (polypeptide) hormones, steroid hormones, and amino acid hormones.

The anterior pituitary gland secretes several crucial hormones, including FSH (follicle-stimulating hormone), TSH (thyroid-stimulating hormone), ACTH (adrenocorticotropic hormone), LH (luteinizing hormone), PRL (prolactin), and GH (growth hormone).

The posterior pituitary gland primarily stores and releases ADH (antidiuretic hormone) and oxytocin. These hormones are synthesized in the hypothalamus.

To exert its action, a hormone must bind to a specific receptor. Receptors are large proteins that are specific for a particular hormone, like a key fitting into a lock. They can be located on the cell surface (protein hormones), in the cytoplasm (steroid hormones), or in the nucleus (thyroid hormones).

Up-regulation occurs when the number of receptors increases, often in response to low hormone levels. Down-regulation occurs when the number of receptors decreases, often in response to high hormone levels. These mechanisms help to regulate the sensitivity of cells to hormones.

This circulatory system connects the hypothalamus to the anterior pituitary. It transports releasing and inhibiting hormones secreted by the hypothalamus, affecting the anterior pituitary's hormone production.

This tract is a nervous connection between the hypothalamus and the posterior pituitary. It carries ADH and oxytocin, synthesized in the hypothalamus, to the posterior pituitary for storage and release.

Protein hormones, acting through non-genomic mechanisms, bind to receptors on the cell membrane. This forms a hormone-receptor complex, which activates a secondary messenger system. Examples include adenyl cyclase-cAMP, the inositol-diacylglycerol system, and cGMP.

Adenyl cyclase, activated by the hormone-receptor complex, catalyzes the conversion of ATP to cAMP. cAMP acts as a second messenger, initiating intracellular signaling pathways.

IP3 and DAG are second messengers generated by the inositol-diacylglycerol system. IP3 triggers the release of Ca++ from intracellular stores, while DAG activates protein kinase C, which phosphorylates proteins.

Steroid hormones, acting through genomic mechanisms, pass through the cell membrane and bind to receptors in the cytoplasm. The hormone-receptor complex then enters the nucleus, interacts with DNA, triggering transcription and mRNA formation, ultimately leading to protein synthesis.

Thyroid hormones (T3 and T4) are small and can easily pass through cell membranes. They bind to receptors within the nucleus, affecting DNA transcription and leading to increased mRNA production and protein synthesis.

Negative feedback is a mechanism that maintains a constant hormone level in the blood. When a hormone level rises, it inhibits the gland's further secretion, thus reducing the hormone level back to normal.

There are three types: short-short loop (ultra-short), short loop, and long loop. In the short-short loop, the hormone inhibits its own secretion. In the short loop, the hormone inhibits its releasing hormone. In the long loop, the hormone inhibits both the hypothalamus and pituitary gland.

Positive feedback occurs when an increase in one hormone stimulates the secretion of another hormone. For example, a rise in estrogen levels during the menstrual cycle triggers a surge in LH, leading to ovulation.

The pituitary gland develops from two sources: Rathke's pouch (roof of the stomodeum) forms the anterior lobe, and the infundibulum (floor of the diencephalon) forms the posterior lobe.

The pituitary has two main parts: the adenohypophysis (anterior lobe) and the neurohypophysis (posterior lobe). The adenohypophysis is further divided into pars distalis, pars tuberalis, and pars intermedia.

The parathyroid glands secrete parathyroid hormone (PTH), which regulates blood calcium levels by increasing calcium reabsorption from bones and increasing calcium absorption in the gut.

The parathyroid gland contains chief cells (principal cells) and oxyphil cells. Chief cells are responsible for PTH production, while oxyphil cells have an unknown function.

The thyroid gland originates from the endoderm of the first pharyngeal arch. A thyroglossal duct descends from the foramen cecum and forms the thyroid acini. The connective tissue and vessels are derived from surrounding mesoderm.

The thyroid gland secretes thyroid hormones, T3 and T4, which regulate metabolism, growth, and development. It also produces calcitonin from parafollicular cells, which lowers blood calcium levels.

Thyroid hormone synthesis involves iodide trapping, oxidation to iodine, binding to thyroglobulin, iodination of tyrosine, coupling of DIT and MIT, and release of T3 and T4.

Thyroid hormones increase basal metabolic rate, oxygen consumption, heat production, glucose absorption, lipolysis, protein synthesis, and vitamin needs.

Thyroid hormones affect the gastrointestinal system (motility and secretion), cardiovascular system (heart rate, contractility, blood pressure), and central nervous system (myelination, synaptic transmission, mental alertness).

Hypothyroidism is a condition caused by insufficient thyroid hormone production. It can be primary (thyroid gland issue), secondary (pituitary issue), or tertiary (hypothalamus issue).

Primary hypothyroidism can be caused by congenital absence of the thyroid gland, chronic iodine deficiency, chronic thyroiditis, excessive antithyroid drugs, or surgical removal of the thyroid gland.

Myxoedema is characterized by a decreased BMR (leading to weight gain, cold intolerance, coarse hair), skin changes (dry, puffy, yellow), CNS slowing, CVS issues, constipation, increased cholesterol, and non-pitting edema.

Cretinism manifests as delayed physical and mental development, dwarfism, disproportionate growth, obesity, slow eruption of teeth, delayed fontanel closure, and increased blood cholesterol.

Hyperthyroidism is a condition caused by excessive thyroid hormone production. It can be caused by acute thyroiditis, thyroid tumors, Graves' disease, excessive thyroid hormone administration, or pituitary tumors.

Hyperthyroidism is characterized by an increased BMR (leading to heat intolerance, weight loss, warm skin), CNS hyperactivity (irritability, tremors, insomnia), CVS changes (tachycardia, increased pulse pressure), exophthalmos (bulging eyes), and gastrointestinal issues (diarrhea, hyperphagia).

A goiter is an enlarged thyroid gland. It can occur with normal, decreased, or increased thyroid function.

Goiters can be classified into physiologic, hypothyroid (colloid goiter), and hyperthyroid (parenchymatous goiter).

Goitergenic substances interfere with thyroid hormone synthesis. Drugs that block iodide trapping, such as perchlorate, nitrate, and iodate, compete with iodine and prevent its uptake. Drugs that block iodine utilization, such as thiourea, thiouracil, and thiocarbmide, compete with tyrosine and inhibit the formation of MIT and DIT. Natural goitergens, like those found in carrots, cabbage, and turnips, contain progoitrin, which is converted to goitrin, interfering with thyroid hormone production.

The pineal gland, a neuroendocrine gland in the brain, secretes melatonin, which regulates circadian rhythms, affects gonadal function, and acts as an antioxidant.