Flashcards in Thyroid hormones Deck (34):
What hormones are secreted by the thyroid gland?
Thyroxine (T4) – 93%
Triiodothyroxine (T3) – 7%
What is the function of the thyroid hormones?
1. Both hormones increase metabolic rate
2. Stimulates oxygen consumption by most cells
3. Regulates lipid and CHO metabolism
4. Influence body mass and mentation
What controls the secretion of the thyroid hormones?
1. Hormone secretion controlled by TSH
2. TSH Secreted by anterior pituitary
What other hormones is secreted by the thyroid gland?
What is the histology of the thyroid gland?
1. Thyroid gland contains multiple follicles; Follicles are lined with simple cuboidal epithelium
2. Follicle cavity contains colloid (Viscous fluid containing dissolved proteins)
What is the function of the epithelial cells?
Epithelial cells synthesize thyroglobulin & secrete it into colloid; Thyroglobulin contains tyrosine (Building block of thyroid hormones)
What are the steps in formation and release of T3 and T4?
1. Follicular cell synthesizes enzymes and thyroglobulin for colloid
2. Iodine is cotransported into the cell with Na and transported into colloid
3. Enzymes add iodine to thyroglobulin to make T3 and T4
4. Thyroglobulin is taken back in to the cell
5. Intracellular enzymes separate T3 and T4 from the protein
6. Free T3 and T4 enter the circulation
7. Most T3 and T4 is then bound to proteins in the bloodstream
What does T4 and T3 bind to in the bloodstream? How much is bound?
1. Majority of TH entering bloodstream attaches to thyroxin binding globulins (TBG)
A. Approximately half of bound T4 is released every 6 days
B. Approximately half of bound T3 is released after 1 day
What happens to T3 and T4 after entering tissue cells?
1. After entering tissue cells, both hormones bind again with intracellular proteins
2. Gradually released and slowly used over period of days or weeks
How are bound and unbound thyroid hormones levels balanced?
1. An equilibrium exists between bound and unbound thyroid hormones
2. Free thyroid hormones are bound to carriers at same rate as bound hormones are released
What proteins bind thyroid proteins? (In addition to TBG)
What is T4 primarily bound to?
1. TBG: 67%
2. Transthyretin: 20%
3. Albumin: 13%
What is T3 primarily bound to?
1. Albumin: 53%
2. TBG: 46%
3. Transthyretin: 1%
If a person is deficient in binding proteins, how can this affect thyroid hormone levels?
Total and bound lab values of thyroid hormones will be low. Free thyroid levels will be normal
What is TRH? Where is it released from? What does it do?
1. Thyrotropin releasing hormone
2. Produced in hypothalamus
3. Stimulates anterior pituitary gland to produce TSH
What are the functions of TSH?
1. Increases proteolysis of thyroglobulin
2. Increases activity iodide pump
3. Increases iodination tyrosine to form thyroid hormones
4. Increases size, # & secretory activity thyroid cells
What glands do the negative feedback system of the thyroid hormones target?
2. Anterior pituitary
Where does T4 get cleaved to T3?
Intracellularly by 5'-deiodinase
What does T3 bind to in the cell? What does it do?
1. Binds Nuclear receptors
2. Initiates transcription of a variety of proteins and enzymes
3. Overall effects: increase metabolic rate, increase oxygen consumption, normal growth and development, central nervous system, increased cardiac output, increase renal function
Which thyroid hormone is more metabolically active?
What are the specific effects of thyroid hormones on metabolism?
1. Increase food intake
2. Increase lipolytic effect on adipose tissue
3. Decrease adipose tissue
4. Decrease muscle mass
5. Increase temperature
What is the pathophys of hyperthyroidism?
1. Metabolic imbalance that results from overproduction of thyroid hormones
What is the most common type of hyperthyroidism?
Most common type is Graves Disease
What are the clinical symptoms of hyperthyroidism?
3. Palpable lymph nodes
4. Heat intolerance
6. Loss of weight
7. Menstrual irregularities
9. Localized myxedema
12. Clubbing of fingers
13. muscular weakness
What is the pathophys of graves disease?
Autoantibodies attach to and stimulate TSH receptors on thyroid gland
What are the complications of grave's disease?
1. Leads to:
Increased stimulation of gland
Increased hormone production
2. Results in:
Increased protein, lipid and CHO catabolism
What is the pathophys of hypothyroidism?
1. Results from:
A. Hypothalamic, pituitary or thyroid insufficiency
B. Resistance to thyroid hormone
2. Includes congenital hypothyroidism
A. Deficiency of thyroid hormone during fetal development and early infancy
What are the primary causes of hypothyroidism?
1. Primary -Most common
A. Thyroidectomy or radioactive iodine
B. Inflammation or goiter
C. Chronic autoimmune thyroiditis: Hashimoto’s thyroiditis
What are the secondary causes of hypothyroidism?
1. Pituitary failure
2. Iodine deficiency
What is the pathophys of Hashimoto's thyroiditis?
1. Hashimoto's thyroiditis is a common form of hypothyroidism and is caused by autoimmune antibodies directed against thyroglobulin or thyroid peroxidase.
2. This results in low TH production (and high circulating TSH) and eventual destruction of the thyroid gland.
What causes congenital hypothyroidism?
1. Congenital lack of thyroid gland
2. Failure of thyroid gland to produce TH
3. Lack of iodine in diet
What is the main characteristic of congenital hypothyroidism?
failure of growth
What are the sxs of a goiter?
1. Pressure on windpipe and esophagus
2. Breathing difficulties and cough
3. Hoarseness and swallowing difficulties