L18-19 Thyroid Flashcards
What regulates thyroid hormone secretion?
Thyroid regulating hormone (TRH) - released from hypothalamus, acts on pituitary
Thyroid stimulating hormone (TSH) - released from pituitary, acts on thyroid
Describe the structure and location of the thyroid gland
A ‘butterfly’ shaped gland at the base of the neck on either side of the trachea
Isthmus connects the two masses in-front of trachea
Name the cells in the thyroid gland. What do they do?
Follicle cells - produce thyroglobulin, and stores it with iodine in follicle lumen as thyroid hormone precursors
Parafollicular cells - produce calcitonin
What is the structure of thyroid hormone (TH)?
Two related compounds: T3 and T4
Both composed of 2 tyrosine molecules and 3 or 4 iodine atoms
What are the functions of thyroid hormone?
Increases metabolic rate and heat production
Regulates growth and development
Maintenance of blood pressure
How are thyroid hormones synthesised?
- Iodide ions are taken in by follicle cells and released into follicle lumen
- Become oxidised and bind to tyrosine in thyroglobulin molecules
- Endocytosed by follicle cells
- Fuse with lysosome to cleave tyrosine from thyroglobulin
- Released into blood
How is thyroid hormone transported?
Thyroxine binding globulins (TBGs)
What is the more active thyroid hormone compound?
T3 - peripheral tissues convert T4 into T3
How is thyroid hormone regulated?
When can this system be overcome?
Negative feedback loop - increased TH inhibits TSH release
TRH can over come inhibition in pregnancy or cold temperatures
Name three thyroid function tests
TSH, FT3 (free T3), FT4, thyroid antibodies, calcitonin thyroglobulin
What is thyroid stimulating hormone (TSH) used as an initial test to evaluate?
Thyroid function and hypo- or hyperthyroidism symptoms
What does an abnormal TSH blood level indicate?
Increase or decrease in available thyroid hormones, but doesn’t give cause
What can a high TSH, low FT4 indicate?
Underactive thyroid (hypothyroidism),
What can a low TSH, high FT4 indicate?
Overactive thyroid (hyperthyroidism), problem with pituitary
What is the free T3 (FT3) test used for?
Aid diagnosis of hyperthyroidism