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Lecture 23: Thyroid Function
Reading: OpenStax A&P Text Chapter 24
Thyroid Histology
ANATOMY: also microscopic view… (Feel your thyroid cartilage of the larynx (Adam’s apple); the thyroid gland is inferior to this)
1 million follicles in gland
Thyroid hormone synthesis
Thyroid Hormone (affects metabolism, growth and development) is produced by follicular cells. There are two forms (T3 and T4)…
T3 is the “active” version
1. Takes place in colloid
2. Cell synthesizes THYROGLOBULIN from tyrosine (AA derivative!)
3. (Thyroglobulins contain 70 tyrosines!)
4. Iodine is pumped into the colloid…Is energy used for this process? Explain…
5. Iodine is then added to the TYROSINE molecules ON the thyroglobulin.
A. Sometimes 1 iodide is added...other times 2 iodides are added
B. Everything is STILL a thyroglobulin molecule…
6. Now the 1 and 2 iodide molecules COMBINE in varying ways…
A. 1+2 = 3 (triiodothyronine, or T3)
B. 2+2 = 4 (tetraiodothyronine, or T4 aka thyroxine)
7. Thyroglobulin is taken into cell through endocytosis
8. T3 and T4 are cleaved off the thyroglobulin molecule
A. If any tyrosine molecules only have 1 or 2 iodines attached, the iodine is cleaved and RECYCLED…imbalance here can
lead to major iodine deficiency
9. Lipid soluble T3 and T4 enter the circulation
A. 93% is T4
B. 7% is T3…this is the ACTIVE FORM…
C. During circulation, T4 slowly loses an I and turns into T3
Mechanism of action
1. Diffuses into body cell…
2. T3 can bind with thyroid hormone receptors on DNA
3. (T4 can also have its extra iodine cleaved within the cell!)
4. This stimulates protein synthesis (and thus increases metabolism, growth, and development)
Control of Thyroid hormones
1. TRH (hypothalamus)
2. TSH (ant pit)
A. Increases chopping of thyroglobulin
B. Increases rate of iodine absorption into colloid
C. Increases rate of iodine added to tyrosine
D. Increases size and activity of thyroid cells
E. Increases number of thyroid cells
3. T3 and T4…
Biol 7: Human Physiology Spring 16
CC-BY Wendy Riggs
Calcium homeostasis
Thyroid gland also produces calcitonin (C cells do this) which is involved in Ca2+ homeostasis…
1. Calcium plays a role in:
A. All muscle contractions
B. Blood clotting
C. Transmission of nerve impulses
D. Hypercalcemia → depressed NS
E. Hypocalcemia → excitable NS
2. Starting parameters:
A. Normal range- 2.4 mmol/L (9.4 mg/dl)
B. 0.1% body calcium is stored in ECF
C. 1% in cells…(concentrated in SR and mitochondria, leaving low conc in ICF)
D. Rest in BONES! They serve as a RESEVOIR for Ca.
Bones as a Ca2+ reservoir
1. Ostoblasts BUILD bone
A. Found on outer surfaces of bone AND bone cavities
B. Note: osteoblast activity is related to “stress” on the bone (Bones actually atrophy when not in use!)
C. Stress can determine shape of the bone by increasing blast activity in stressed areas.
D. Connect this to osteoporosis (when bone reabsorption is greater than bone deposition)!
E. When an osteoblast gets trapped in matrix, it is considered an OSTEOCYTE…but it doesn’t die (since it also laid down
canals for BLOOD SUPPLY as it was building!)
2. Osteoclasts reabsorb bone
A. Huge multinucleate cells
B. Related to monocytes!
C. Secrete acids and enzymes that dissolve bone
Parathyroid hormone (PTH)
Hormones involved in bone growth/Ca homeostasis:
1. Produced by parathyroid glands and essential for life (in rabbits and dogs…ahem)…
2. Low Ca2+ levels trigger receptors on the parathyroid chief cells (oxyphil cells have unknown function), leading to PTH
3. Function: increase plasma Ca2+ conc
4. Mechanism: Increases osteoclast activity, increases renal reabsorption of calcium, and increases intestinal absorption of Ca2+ by
increasing CALCITRIOL…
Calcitriol (Vitamin D3)
1. Produced in liver and kidneys
2. Function: Enables increased absorption of Ca in intestines
3. Mechanism:
4. Increases the production of “Calcium-binding protein” in the luminal membrane of intestinal epithelium
1. Produced by the C cells in the parafollicular tissue in the thyroid gland (not a huge player though...)
2. Function: Decreases plasma Ca2+ conc
3. Mechanism: Decreases activity of osteoclasts, decreases formation of new osteoclasts
Biol 7: Human Physiology Spring 16
CC-BY Wendy Riggs
Lab 23: Metabolism and Exercise
(iWorx HE-1)
Biol 7: Human Physiology Spring 16
CC-BY Wendy Riggs
External Brain 23: Thyroid Function
1. Know all thyroid gland anatomy as discussed in class.
2. Be able to identify thyroid tissue and label follicles, follicular cells, C cells and colloid.
3. What is the specific mechanism by which thyroid hormone is produced? Your answer should include the specific location(s) of
each step in the process.
4. What stimulates the production of thyroid hormone? Your answer should include the hypothalamus and the pituitary. Also
include all feedback involved in the process.
5. What is the “active” version of thyroid hormone?
6. What is the relative percentage of T3 and T4 released from the thyroid gland? How does this change over time?
7. How does thyroid hormone affect tissues?
8. What does TSH do? Be specific.
9. Describe general characteristics caused by hypothyroidism.
10.Describe general characteristics caused by hyperthyroidism.
11. What is a goiter?
12.Explain how a goiter can be caused by both hyperthyroidism and hypothyroidism.
13.Describe the importance of calcium ions. What are they involved in?
14.What is the relative amount of the body’s calcium that is found in the cells and ECF? Where is the rest of the calcium found?
15.Compare and contrast the 3 types of bone cells.
16.Compare and contrast the 3 hormones that are involved in calcium homeostasis.
Biol 7: Human Physiology Spring 16
CC-BY Wendy Riggs