The Major Peripheral Endocrine Glands

• Thyroid Gland is a two-lobed endocrine structure located below the larynx, with a narrow middle portion that gives it a butterfly or bowtie shape.
• Major secretory cells of the Thyroid Gland are the follicular cells, which are arranged into hollow spheres called follicles and contain a colloid substance that serves as a storage site for thyroid hormone.
• Follicular cells contain two iodine-containing hormones, derived from tyrosine, tetraiodothyronine (T4) and triiodothyronine (T3).
• Thyroid hormones exert their effect throughout the body and most body cells have receptors for thyroid hormones.
• Increase the basal metabolic rate (BMR) and result in increased metabolism of carbohydrates, lipids, and proteins.
• Thyroid hormones maintain normal body temperature and increase the rate of protein synthesis, rates of glucose, and fatty acid breakdown to produce ATP.
• Parathyroid Gland is partially embedded in the thyroid gland and its majority of the cells are called chief cells or principal cells.
• Parathyroid cells produce parathyroid hormone (PH or parathormone), which is a regulator of the levels of calcium, magnesium, and phosphate ions in the blood.
• Parathyroid hormone increases the number and activity of osteoclasts, bone cells that degrade bone to initiate remodeling.
• High activity of osteoclasts results in the release of calcium and phosphate ions in the blood.
• Parathyroid hormone acts on the kidneys to slow the rate of calcium and magnesium ions losses via urine.
• Adrenal Gland is paired and pyramidal endocrine structures located above each kidney and during embryonic development, these glands differentiate into two functionally different regions: Adrenal cortex and Adrenal medulla.
• Cells of the adrenal cortex produce three sets of hormones: Mineralocorticoids, Glucocorticoids, and Weak androgens.
• Mineralocorticoids, such as Aldosterone, regulate levels of sodium and potassium ions and increase the reabsorption of sodium ions and the secretion of potassium and hydrogen ions via the kidneys.
• Glucocorticoids, including the hormone cortisol, increase the rate of protein breakdown, glucose formation, and fat tissue breakdown and increase an individual’s resistance to stress via the glucose supplied by the liver.
• Weak androgens, such as DHEA, increase the growth of underarm and pubic hair in both males and females.
• Adrenal Medulla is a modified ganglion of the ANS and synthesizes the hormones epinephrine and norepinephrine (adrenaline and noreadernaline).
• During stressful or fight-or-flight situations, the adrenal medulla receives impulses from the hypothalamus and secretes epinephrine and norepinephrine.
• Epinephrine and norepinephrine increase the heart rate and output of the heart, increasing blood pressure and increasing the rate of blood flow in some organs such as the heart, liver, skeletal muscles, and adipose tissue.
• Epinephrine and norepinephrine also increase glucose and fatty acid levels in the blood.
• Pancreas is a mixed-function organ, both endocrine and non-endocrine functions, with some exocrine glands of the pancreas producing and secreting various digestive enzymes.