Endocrine System: Hormones and Regulation

General Characteristics of the Endocrine System

• The endocrine system functions as a control system to maintain homeostasis.
• It is a key regulator of energy use, metabolism, growth, and reproduction.

Endocrine vs. Nervous System

• The endocrine system is slower, and travels short or long distances
• The nervous system is fast-acting and travels short distances
• The endocrine system regulates internal enviroments
• The nervous system regulates both internal and external environments

Hormones

• Hormones are essential for communication and regulation within the body.

Amino Acid Derivatives

• Derived from a single amino acid.
• Examples include T4, T3, epinephrine, and norepinephrine.

Peptide Hormones

• Peptide hormones are chains of amino acids .
• Examples include polypeptides like ADH and oxytocin.
• Small proteins like insulin, GH, and PRL are peptide hormones.
• Glycoproteins such as TSH, LH, and FSH are peptide hormones.
• Lipid derivatives consist of carbon rings and side chains from fatty acids or cholesterol.
• Eicosanoids, including prostaglandins, are peptide hormones.
• Steroid hormones are built from cholesterol molecules, such as testosterone, estrogen, progesterone, and corticosteroids.

Chemistry of Hormones

• Hormones can be steroid (lipid-soluble, derived from cholesterol) or non-steroid (water-soluble).
• Steroid pass easily through the cell membrane.
• Non-steroid hormones require a receptor on the target cell membrane.

Steroid Hormone Action

• Steroid hormones enter cells through the cell membrane.
• They bind to a protein receptor in the cytoplasm.
• The hormone-receptor complex affects gene expression.
• New proteins execute functions.

Non-Steroid Hormone Action

• Non-steroid hormones include amines, peptides, proteins, and glycoproteins.
• They bind outside the cell and act as the first messenger.
• Second messenger activates a protein cascade.
• G-protein regulation uses cyclic AMP.

Regulation of Hormone Secretion

• Hormone secretion is regulated through a combination of mechanisms
• Hormonal controls involve tropic hormones from the hypothalamus.
• The autonomic nervous system provides nervous control.
• Humoral control involves regulating blood levels.

Pituitary Gland and Hypothalamus: Anterior Pituitary

• The anterior pituitary controls growth, milk production, and hormone secretion.
• Human Growth Hormone (HGH) promotes the body's growth and targets bone, muscle, and adipose tissue.
• Prolactin (PRL) stimulates milk production by the mammary glands and targets the mammary glands.
• Thyroid Stimulating Hormone (TSH) secretion from thyroid gland and targets the thyroid gland.
• Adrenocorticotropic Hormone (ACTH) controls hormone secretion by the adrenal cortex
• ACTH targets the outer portion (cortex) of the adrenal gland.
• Follicle Stimulating Hormone (FSH) response varies by sex
• FSH causes maturation of an ovarian follicle and ovum in females.
• FSH causes maturation of sperm in the testes in males.
• Luteinizing Hormone (LH) response also varies by sex.
• LH causes ovulation in females and stimulates secretion of testosterone in males.

Pituitary Gland and Hypothalamus: Posterior Pituitary

• The posterior pituitary secretes two hormones: antidiuretic hormone (ADH) and oxytocin (OT).
• ADH targets distal convoluted tubules (DCT) in the kidney, causing reabsorption of water into the bloodstream and increasing blood pressure.
• Oxytocin (OT) targets uterine smooth muscle and breasts.

Pituitary Hormone Imbalances

• Pituitary Dwarfism results from hyposecretion of GH during growth years, leading to slow bone growth, early closing of epiphyseal plates, and short stature; treatment involves oral GH therapy.
• Pituitary Gigantism results from hypersecretion of GH during growth years, leading to excessive long bone growth.
• Acromegaly is hypersecretion of GH during adulthood that leads to Bones of hands, feet, cheeks, and jaw thicken and soft tissues also grow leading to enlarged bones and soft tissues.
• Diabetes Insipidus results from hyposecretion of ADH and leads to diuresis, dehydration, and thirst. It can be treated with ADH nasal spray.

Thyroid Gland

• Located below the larynx and around the trachea and is involved in iodine uptake.
• The thyroid gland produces thyroxine (T4) and triiodothyronine (T3).
• The thyroid gland produces 3 hormones when stimulated by TSH: thyroxine, trriiodothyronine and calcitonin.
• Thryoxine and triiodothyronine increases basal metabolic rate by stimulating cellular oxygen, therefore producing ATP.
• Calcitonin release is triggered by elevated blood calcium levels.
• Calcitonin then targets bone cells to stimulate osteoblast activity and inhibit osteoclast activity, while also targeting DCT's to cause calcium and phosphate levels back to normal.

Thyroid Hormone Imbalances

• Cretinism results from hyposecretion of T3/T4 during fetal life and infancy. It can be prevented with newborn testing. Treatment can involve oral thyroid therapy.
• Myxedema results from hyposecretion during adulthood. oral thyroid hormones can reduce the symptoms.
• Grave's disease is an autoimmune disorder, and causes growth of the thyroid with no negative feedback. Treatment involves surgery, anti-thyroid drugs etc.
• Goiter is a result of low thyroid hormones due to iodine deficiency. No thyroid hormones inhibit TSH, and the thyroid is still overstimulated.

Parathyroid Gland

• There are 4 small glands are embedded in the posterior of the thyroid.
• These glands produce parathyroid hormone where release is stimulated by decreased blood calcium levels.
• PTH targets bone cells and proximal convoluted tubules.
• PTH bone cells and proximal convoluted tubules activate bone cells to resorb bone.
• PTH cells reabsorb calcium back into the blood stream promoting calcium absorption.
• PTH causes an increase in blood calcium and phosphate levels back to normal.

Parathyroid Hormone Imbalances

• Hyperparathyroidism is usually the result of a tumor in the thyroid which may result in loss of bone tissue. leads to hypercalcemia.
• Hypoparathyroidism is less common. results in weakness and faulty muscle contractions.

Adrenal Gland

• The adrenal gland is located on top of the kidneys.
• The outer cortex which secretes aldosterone, cortisol, corticosterone, cortisone, estrogen and supplemental sex hormones
• and the inner medulla which synthesises epinephrine and nor-epinephrine.

Hormones of the Adrenal Medulla

• The adrenal medulla is located in the central portion of the adrenal glands.
• Produces epinephrine and norepinephrine and these hormones target the heart.
• The adrenal Medulla produces smooth muscle, peripheral arterioles, terminal bronchioles.

Hormones of the Adrenal Cortex

• The cortex is located in the outer portion of the adrenal glands.
• Produces steroid hormones when stimulated by Aldosterone, Cortisol etc. and supplemental sex hormones that target sex organs.

Adrenal Cortical Hormone Imbalances

• Addison's Disease involves hyposecretion of AC hormones due to autoimmunity or infectious disease. This results in mental lethargy, anorexia, nausea, vomiting, Na+, dehydration, K+, BP etc.
• Cushing's Syndrome involves hypersecretion of AC hormones due to pituitary tumor or steroid administration. Redistributes fat, thin limbs, etc.

Pineal Gland

• Located in the brain stem thalamus.
• It secretes melatonin during daylight(circadian rhythm) which can affect moods, emotions, etc.

Gonadal and Placental Hormones

• Ovaries secrete estrogen and progesterone.
• estrogen causes hair follicles, breasts, etc. and progesterone prepares the uterine lining for implantation of zygote.
• Testes secrete testosterone which causes hair follicles in facial, vocal chords etc.

Endocrine Pancreas

• Located behind the stomach.
• Alpha cells produce glucagon which stimulates secretion and blood glucose levels.
• Beta cells produce Insulin which stimulate increased blood glucose levels and decreases blood glucose levels.

Endocrine Pancreas: Imbalances and Diabetes Mellitus

• Diabetes Mellitus presents with symptoms of polyuria, polydypsia, and polyphagia. The two major types are Type 1 and Type 2
• Type I is an insulin deficiency.
• Type II is when patients are overweight with hypertension. It is controlled with weight loss.

Organs with Secondary Endocrine Function

• Heart secretes Atrial natriuretic peptide(ANP) to reduce blood volume
• Kidneys secrete Renin to stimulate the release of aldosterone.
• Liver secretes insulin-like growth factor to stimulate insulin and Hepcidin that block the release of iron into body fluids