Chapter 17 Part 2

Thyroid gland

• located in the beck where it wraps around the trachea

• medial region: isthmus

• two lateral lobes- the parathyroid gland is embedded in the lateral lobes posteriorly (PTH production)

• produces thyroid hormones (T3 and T4) and calcitonin

thyroid tissue cells?

Follicular cells- make up most of thyroid tissue responsible for thyroid hormones production.

• hormone production occurs in the central cavity of follicles called the colloid

Parafollicular cells- tissue between follicular cells, produce calcitonin

Thyroid hormones production

regulation through hypothalamus-pituitary gland axis

• TRH release from hypothalamus

• TSH release from the pituitary

• Triiodothyronine T3 / Thyroxine T4 release from thyroid gland

• mediated through thyroglobulin (Tg)

Thyroglobulin (Tg)

thyroglobulin is used to produce T3/T4 when combined with iodide in the colloid of thyroid follicles

thyroid hormone production in follicles

1. an epithelial cell takes up iodide from the blood

2. the cell synthesizes thyroglobulin from tyrosine

3. thyroglobulin is secreted into the follicle lumen and iodinated

4. thyroglobulin is taken up by endocytosis

5. lysosomal enzymes digest thyroglobulin into t3 and t4

thyroid hormones functions?

• increases the basal metabolic rate of body cells

• raises body temperature

• stimulates red blood cell production and enhances oxygen delivery

• elevates rates of oxygen consumption and energy consumption in cell

• stimulates the activity of other endocrine tissues

• induces turnover of minerals in bone

• increases heart rate and blood pressure

• increases sensitivity to sympathetic stimulation

regulation of thyroid hormones levels

• low t3/t4 in blood- TRH release from hypothalamus, TSH release from pituitary gland, T3/T4 release from thyroid gland

• High T3/T4 in blood
  - High T3/T4 levels inhibit TRH and TSH release

calcitonin

• released from parafollicular cells in response to a rise in blood calcium levels

• brings calcium into normal range by - inhibiting the activity of osteoclasts
  - increasing osteoblastic activity
  - decreasing calcium absorption in the intestines
  - increasing calcium loss in the urine

hypothyroidism

• decreased blood levels of T3 and T4 when the follicular cells within the thyroid gland are failing to secrete adequate levels of T3 and T4

• can be congenital (underdeveloped thyroid))

• can be due to iodine deficiency

symptoms and treatment of hypothyroidism

• cold intolerance

• weight gain

• dry and thickened skin

• fatigue

• constipation

• bradycardia

• hypoglycemic symptoms

• treatment- Supplementary T3 and T4 hormones

dietary iodine deficiency

• results in enlarged thyroid gland

• low T3 and T4 production induces TSH secretion

• TSH causes thyroglobulin production (precursor for T3 and T4)

• colloid deposits to induce thyroid hormone(T3 and T4) production

• hormones cannot be produced due to lack of iodine

• colloid accumulation increases thyroid gland size , a condition called goiter

hyperthyroidism

• increased blood levels of T3 and T4 when the follicular cells

hyperthyroidism symptoms and treatments?

parathyroid gland?

• contains two twos of cells

• Parathyroid cells- produce parathyroid hormones (PTH)

• Oxyphil cells- role unclear

Parathyroid hormones (PTH)?

• secreted when blood calcium is low

• increases osteoclast activity

• increases calcium reabsorption in kidney

• increases production of calcitriol (vitD) in kidneys

• calcium increases absorption of dietary calcium in intestines

Regulation of blood calcium levels with PTH and calcitonin

• two mechanisms constantly maintain blood calcium concentration at homeostasis

• Parathyroid hormone increases blood
  calcium levels when they drop too low

• Calcitonin decreases blood calcium
  levels when they become too high

Hyperparathyroidism

PTH overproduction high activity of PT glands or enlargement

- Often from parathyroid tumor
- Leads to hypercalcemia, deformities
- calcium accumulates in tissues and organs impairs function
- increased calcium causes a decrease in body’s sensitivity to sodium, neuronal responses are reduced
• Bone/joint pain, osteoporosis
• Lethargy, muscle weakness, excessive urination, kidney stones
- Usually treated with surgery

Hypoparathyroidism

Typically caused by accidental injury/removal during surgery
- Leads to hypocalcemia
- low blood calcium increases membrane permeability to sodium, resulting in muscle twitching, cramping, spasms, or convulsions

• Muscle cramps/spasms, reduced kidney function, heart arrythmia

• In children, can lead to malformed teeth or stunted growth

Treated with calcium/vitamin D supplements
• synthetic PTH

Adrenal gland

• sit on top of the kidneys (suprarenal)

• cortex (outer layer)

adrenal gland hormones

• cortex and medulla produce different hormones

• Adrenal medulla produces stress hormones such as epinephrine, norepinephrine

• The cortex is further subdivided into three zones, and each zone produces different types of hormones

what are the three zones and the hormones they produce in the adrenal cortex?

1. Zona glomerulosa (outer) – releases mineralocorticoids, principally aldosterone, which controls electrolyte balance in the kidneys.
2. Zona fasciculate (middle) – produces glucocorticoids such as cortisol and cortisone which influence metabolism of glucose, protein, and fat;controlled by ACTH.
3. Zona reticularis (inner) – produces androgens or adrenal sex hormones such as testosterone which influence masculinization.

Aldosterone

• released from the most superficial region of the adrenal cortex

• released in response to angiotensin II hormones

• in response to elevated low salt, low blood pressure or low blood volume

• increases blood sodium levels by increasing sodium retention and water retention

Cortisol

• released from intermediate region of adrenal cortex in response to stress

• Primary function: promotes catabolism of glycogen to glucose, stored triglycerides into fatty acids and glycerol, muscle proteins into amino acids

• Secondary function: suppresses immune function
  - cortisone injection to inflamed joints
  - hydrocortisone cream for rashes
  - prednisone tablets

Androgens

• released the deepest region of the adrenal cortex

• produced in small amounts, supplement gonadal androgens

• during adulthood most androgens are produced in the gonads

• androgens are typically converted to estrogen and testotersone in the gonads

• - adrenal androgens become the main source of androgens in
  postmenopausal women when ovary function declines

Epinenphrine and norepinephrine

• produced by unique post-ganglionic SNS neurons called chromaffin cellls that makes up the adrenal medullary tissue

• - cause glycogen breakdown into glucose in liver/muscle
  - resulting in increased blood glucose levels
  - increase the heart rate and blood pressure
  - dilate the airways raising blood oxygen levels
  - cause vasodilation while triggering vasoconstriction to blood vessels
  serving the gastrointestinal tract, kidneys, and skin
  - downregulate some components of the immune system
  - cause loss of appetite, pupil dilation, loss of peripheral vision

Addison’s disease

• Immune system dysfunction that causes the immune system to attack the adrenal glands

  • Other causes include infections and cancer

Addison’s disease symptoms and treatments?

Symptoms include
- Weight loss
- Muscle weakness
- Fatigue
- Low blood pressure
- Patchy skin

• Patients need to take hormone pills to supplement missing adrenal hormones
  for the rest of their lives

Pineal Gland

• part of the epithalamus- contains intrinsically photosensitive retinal ganglion cells (ipRGC)

• produces melatonin

• circadian effector hormones

• daily and seasonally

• high in night

• important for sleep and wakefulness, hibernation, seasonal reproduction of animals

the photoneuroendocrine pathway

• melatonin is inhibited during the day is produced in the night due to the photosensitivity of the neurons

• ipRGCs will send inhibitory signal to SCN when exposed to light

• ipRGCs will send stimulatory signal to SCN when there is no light

• SCN of hypothalamus stimulate superior cervical ganglion in sympathetic trunk (SCG)

• SCG send signals to pineal gland for
  hormone production

what sets circadian clock?

SCN with hypothalamus

What are the two effects that melatonin has to induce sleep?

• inhibits SCN communication to hypothalamus

• lowers body temperature and blood pressure to prepare for sleep

the circadian rhythem

• 24 hour cycle of wakefulness and sleep

• daily melatonin secretion in the night

Pancreas

Exocrine function: Acinar cells secrete digestive enzymes- transported into the small intestine by the pancreatic duct

Endocrine function: Alpha cells secrete glucagon. Beta cells secrete insulin. Two hormones regulate the rate of glucose metabolism in the body.

Glucagon

• made in alpha cells of pancreas

• released in response to hypoglycemia or sympathetic stimulation, increases blood glucose levels

• Glucagon stimulates cells to release glucose into the blood

Insulin

• made in beta cells of pancreas

• secreted in response to hyperglycemia and parasympathetic stimulation

• Insulin stimulates cells to remove glucose from the blood

• promotes glycogen and fat storage

Diabetes mellitus

Metabolic disruption caused by insulin hyposecretion or hypoactivity

Type I

• insulin dependent

• Insulin deficiency from destruction of beta cells
  -Juvenile onset

Type II

• insulin resistant

• Target cells display reduced insulin sensitivity

• Glucose unable to be absorbed

• Adult onset

Diabetes Mellitus pathway?

• Glucose accumulation in blood causes advanced glycation end products that bind
  and alter protein function outside and inside cells
  - Endothelial damage- poor circulation, less oxygenation, tissue damage, kidneys
  and small vessels affected
  - Poor circulation- inflammation is impaired, cuts in limbs prone
  - Impaired vasodilation- atherosclerosis, heart tissue affected

Three major signs of diabetes mellitus

Polyuria – high urine output
Excess glucose causes more water to be excreted

Polydipsia – intense thirst
Excessive water loss causes fluid imbalance

Polyphagia – excessive hunger
Cells unable to access blood glucose
Higher dependence on fat catabolism can cause buildup of ketones

Gonadal hormones

• testes, ovaries, placenta

Ovaries

Produce estrogen (development of secondary sex characteristics) and progesterone
(prepares the uterus for implantation)

Testes

Secrete testosterone (development of secondary sex characteristics, normal sex
behaviors, and production of sperm) and inhibin (inhibits the release of FSH and GnRH
when sperm counts are high.)

Placenta

• a temporary organ only formed during pregnancy.

• Produces hCG hormone (human chorionic gonadotrophin) for maintaining pregnancy and keeping the corpus luteum intact.

Thymus

• small gland- large at birth, grows until puberty- shrinks with age

Endocrine function:
secretes thymopoietin, thymosin, and thymulin (for T cell production)

Immune function: maturation site for T
lymphocytes

what produces thyroid hormones (t3 and t4)

Thyroglobulin is combined with iodide ions absorbed from the diet to form the thyroid hormones.