Thyroid Physiology Flashcards
What external factor is needed to produce thyroxine hormones?
Iodine
What is the most biologically active thyroid hormone?
What is the most common?
T3 - tri-iodothyronine
T4 - thyroxine (pre-cursor to tri-iodothyronine)
What is the main binding globulin for thyroid hormones?
How is the majority of thyroid hormones transported in the body?
In what form can thyroid hormones enter cells?
Thyroxine binding globulin (TBG)
Using TBG
Only in free form can they enter cells
What enzymes break down T3 and T4?
What one breaks down the majority? Where is it found?
De-iodinase enzymes
D3 - found in the brain (not the pituitary)
What is the average weight of the thyroid gland?
25g
What is the main regulator of thyroid hormone release?
TRH - released by the hypothalamus
Describe the negative feedback loop that T3 and T4 are involved in?
When T3 and T4 levels get too high -> inhibit secretion of TSH and TRH
T/F? Thyroxine has no effect on cardiac muscle?
False
Thyroxine causes increase in RR and HR to cater for the increased oxygen requirement of cells due to increased BMR
What is the main effect of thyroid hormones?
They increase BMR (basal metabolic rate)
Briefly describe the 6 steps of thyroid hormone synthesis
- Thyroglobin synthesis (in follicular cells)
- Uptake and concentration of iodide ( in colloid)
- Oxidation of iodide to iodine
- Iodination of thyroglobulin
- Formation of MIT (T3) and DIT (T4)
- Secretion
Describe the histology of thyroid
Colloid surrounded by follicular cells (cuboidal epi cells)
In space between is parafollicular cells
Like little follicles
What are the parafollicular cells also known as?
What do they secrete?
C-cells
Calcitonin - reduces blood calcium levels
Where does the majority of T4 get converted to T3?
Liver
Where are thyroid hormone receptors found?
In the nucleus of cells so have to be able to enter
