The Endocrine System: Thyroid and Parathyroid Flashcards
Thyroid: Metabolic regulator
Thyroid Gland Anatomy
Two lobes connected by isthmus
The thyroid is important for…
metabolism
Thyroid: Metabolic regulator
Thyroid Gland Organization
Outer capsule
Dense CT
Parathyroid glands
Lobules separated by CT Septa
Follicles - parenchyma
Thyroid: Metabolic regulator
Describe the follicles of the thyroid gland.
Lined with simple epithelium
Filled with colloid
Thyroid: Metabolic regulator
What is the structural difference between active vs. inactive thyroid follicles?
Inactive: squamous
Active: cuboidal to columnar
Thyroid: Metabolic regulator
What is colloid?
Thyroglobulin - the storage form of thyroid hormone
Large glycoprotein with no hormone activity
140 tyrosine residues used to make thyroid hormones
Thyroid: Metabolic regulator
Thyroid Stimulating Hormone (TSH)
Produced by pituitary in response to TRH from hypothalamus
Induces all phases of thyroid hormone production
Thyroid: Thyroid Hormone Production
Where is Thyroglobulin produced? What happens to it after production?
Produced in RER
Glycosylated
Exocytoses into the follicular lumen
Thyroid: Thyroid Hormone Production
Iodide from blood
Transported by symporter
Oxidized after released into lumen
Thyroid: Thyroid Hormone Production
Iodination of Thyroglobulin
- Anion transporter, Pendrin, transports iodide to lumen
- At apical surface, iodide is oxidized to iodine by thyroid peroxidase
- In follicle lumen, tyrosine residues of thyroglobulin are iodinated by covalent binding of 1-2 iodine molecules
- Iodinated tyrosines are conjugated to produce mature hormones
Thyroid: Thyroid Hormone Production
Release of T3 and T4
- Iodinated thyroglobulin is absorbed into follicular cells through endocytosis
- Vesicles fuse with lysosomes, and are then transported to basal region
- T3 and T4 are freed from thyroglobulin and released from basal surface into capillaries
Thyroid: Thyroid Hormones
What hormones are produced in the thyroid?
T3 and T4
Calcitonin
Thyroid: Thyroid Hormones
What is the function of T3 and T4?
they act on nuclear receptors
Important for growth, cell differentiation, basal metabolic rate, and oxygen consumption
Thyroid: Thyroid Hormones
What do T3 and T4 affect the metabolism of?
Carbohydrates
Lipids
Proteins
Thyroid: Thyroid Hormones
Calcitonin opposes…
PTH action
Thyroid: Thyroid Hormones
What happens to T3 and T4 after release from the thyroid?
T3 and T4 release → target organs: ↑Growth, development, metabolism, body heat production
T3 and T4 release → T4 → Liver and Kidney → T4 converted to T3
T3 and T4 release → T3 crosses BBB → T3 inhibits hypothalamus
T3 and T4 release → feedback → inhibits TSH from pituitary to reduce T3 and T4
Thyroid
Where are Parafollicular Cells found? What do they contain?
C Cells
Found between follicular cells and basement membrane
Contain granules of calcitonin (release calcitonin)
Thyroid
What is the function of calcitonin?
Inhibits osteoclast activity
What induces the release of calcitonin?
High blood calcium
Parathyroid
Parathyroid Anatomy
Found in the posterior capsule of the thyroid
Variation in location: embryonic migration differences
Parathyroid
Parathyroid Organization
Chief cells (hormone producing - PTH) - smaller and darker staining
Oxyphil cells - large eosinophilic cells
Reticular fibers support chief cells
Parathyroid
How is PTH Release regulated?
Chief cells have a calcium receptor that detects blood calcium levels
When calcium binds, inhibits PTH Release
If calcium levels are low, PTH release proceeds
Parathyroid
What is the default state of chief cells?
Default state is to release PTH; if Calcium levels are high enough, it inhibits PTH release
Parathyroid
Parathyroid cells release…
PTH
Parathyroid
Parafollicular cells release…
Calcitonin
Parathyroid
What is the difference between PTH and Calcitonin?
PTH → ↑ blood calcium in response to ↓ calcium
Calcitonin → ↓osteoclast activity in response to ↑ blood calcium → ↓ blood calcium
Parathyroid
What are the three mechanims to increase blood calcium?
- Degredation of bone by osteoclasts
- ↑ calcium absorption in kidneys
- ↑ calcium absorption in GI Tract
Parathyroid
What is parathyroid hormone (PTH)?
Polypeptide regulator of blood calcium
Parathyroid
What is the function of PTH in bones?
Targets osteoblasts to produce osteoclast stimulating factor
Osteoclasts absorb bone tissue, releasing calcium and phosphate
Parathyroid
What is the function of PTH in the kidneys?
Reduces phosphate levels by reducing phosphate absorption in renal tubules - and increases calcium absorption
Stimulates vitamin D synthesis, increasing GI calcium absorption
Parathyroid
What is the cell types found in the Parathyroid?
Oxyphil cells
Chief cells
