Thyroid physiology Flashcards
Where are thyroid receptors found?
In virtually all tissues
What is the importance of thyroid hormone?
Maintenance of energy homeostasis and regulating energy expenditure
What are the physiological effects of thyroid hormone?
Stimulates cell metabolism and activity
What is the most common preventable cause of brain damage?
Iodine deficiency, causing a lack of thyroid hormone
What is the innervation of the thryroid?
• Sympathetic innervation: vasomotor but not secretomotor
What is the blood supply to the thyroid?
Bilateral superior and inferior thyroid arteries
What are the three, bilateral veins to the thyroid?
superior, middle and inferior
True or false: when the thyroid gland is very large can hear noise from gland (bruit)
True
What is the functional unit of the thyroid gland?
Thyroid follicles
What type of epithelium surround thyroid colloid?
Simple cuboidal
What is the main constituent of thyroid follicles?
Thyroglobulin
What are the two storage forms of thyroglobulin? Which is more active?
T3 and T4, with T3 the more active/potent one
What is the role of the parafollicular cells in the thyroid?
Secrete Calcitonin
Most of the iodide in the body is stored in the thyroid in the form of what?
iodothyronines
How big of a store of Iodine does the thyroid contain?
2 months worth
What is the regulatory mechanism that occurs when plasma iodine levels fall?
Kidneys will hold onto more
• People with diets deficient in iodine can form what?
iodine deficiency goiter
Thyroid hormone is synthesized where?
In the colloid of the thyroid
What transports iodide into the thyroid epithelial cells?
Na/I symporter
Expression of the gene for NIS is inhibited by what and stimulated by what?
Expression of the gene for NIS is inhibited by iodide and stimulated by TSH.
Process of concentrating iodide in the cell is called what?
Iodide trapping
What happens to iodide when it enters the colloid?
Immediately oxidized to iodine and attached to tyrosine
What is the transporter that transports iodide from the apical membrane to the lumen of the follicle?
Pendrin
What is the storage molecule for Iodine? What is the amino acid that this molecule is made up of?
Thyroglobulin, made up of Y residues
• The process of binding iodine with the thyroglobulin is called what?
Organification
What is the enzyme that is responsible for the formation of MIT, DIT, T3 and T4 from thyroglobulin and iodine?
Thyroid peroxidase (TPO)
T3 formation is favored (relative to T4) under what conditions?
Low iodide
What are the steps of secretion of thyroglobulin? What is the receptor utilized to bring T3 back into the epithelial cell from the colloid?
Binding of thyroglobulin to megalin, then endocytosis and lysosomal degradation in the thyroid epithelial cell
Thyroxin = ?
T4
What is RT3? Is it active?
Reverse T3, where the Iodides are distributed in the opposite way as in regular T3.
This is inactive.
What is the molecule release from the hypothalamus that plays a role in the regulation of the thyroid? What does it do?
Thyroid releasing hormone (TSH) stimulates thyrotrophs to release TSH
What is the feedback mechanism for TSH?
T3 feeds back into the anterior pituitary and the hypothalamus to inhibit TSH production
What is the function of TSH? How does it do this?(3)
Stimulates the release of T4 and T3
- Stimulates the iodide pump
- Increases iodination of Y
- Increase thyroid cells
What is the effect of stress on secretion?
Inhibited
What is the half-life of TSH?
~60 minutes
What causes a goiter?
TSH will stimulate hyperplasia of the thyroid
How is TSH transported in the blood?
Bound to plasma proteins–specifically, thyroxine binding globulin (TBG)
What is transthyretin (TTR)?What two things does it bind to?
an additional semi-specific thyroid-binding protein found in the plasma, that can also bind retinol
What is the form of T3/T4 that actually interacts with target cells?
Free form
What predominates in the circulation: T3 or T4?
T4
What is the purpose of having T3/T4 bound to proteins in the plasma? (2)
Serves as a store, and prevents loss through urination
What are the receptors that take up T3/T4 into target cells? Where on/in the cell are they located? Are they energy dependent?
Thyroid hormone receptors located in the nucleus, bound to DNA.
These are energy dependent
Where is the thyroid hormone receptor bound?
To the Thyroid hormone response element on the cell’s DNA
Deficiency of thyroid hormone in kids causes what?
Failure of body growth, and MR
What is cretinism?
Disease caused by extreme hypothyroidism early in life
What is the hormone that converts T4 to T3?
Iodinase
How long does it take for T3/T4 to take effect (relatively)?
Long time
What does hypothyroidism do to plasma [cholesterol]?
Increases
What does thyroid hormone do to cardiac output?
Increases
What is the clinical measurement of thyroid function?
TSH
Why is [TSH] high in hypothyroidism, and low in hyperthyroidism?
TSH will be regulated via feedback inhibition
What happens to the number of beta receptors on the heart with increases in TSH?
Increases
What are the receptors that are stimulated in Grave’s disease?
TSH receptors o the thyroid
Which causes menstrual irregularities: hyper or hypothyroidism?
Hyper
Which causes osteoporosis: hyper or hypothyroidism?
Hyperthyroidism
What are the lab values ([TSH], [T4], [T3]) for someone with hyperthyroidism?
[TSH] = low
[T4], [T5] = high
What is Hashimoto’s thyroiditis?
Type of hypothyroidism where there are autoimmune antibodies directed against thyroglobulin or thyroid peroxidase
Is Hashimoto’s thyroiditis hyper or hypothyroidism?
Hyper first if there is an acute immune response. This will be followed by hypothyroidism.
If there is no acute response, could be mild hyperthyroidism, followed by long term hypothyrodisim
What is the difference between primary and secondary hypothyroidism? Tertiary?
Primary = prob w/ thyroid Sec = prob with pituitary Tert = Hypothalamus prob
What is Myxedema
deposition of mucopolysaccharides in the dermis, which results in swelling of the affected area. This is seen in hypo/hyperthyroidism
