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
Why are free T3 and T4 levels measured to diagnose thyroid dysfunction instead of total T3 and T4?
Total T3 and T4 levels can be affected by medications such as oral contraceptives and aspirin
In what cases are FT3 levels not used as an indicator of thyroid function? Why is this?
Hospital patients - most people in hospital will have low FT3 levels as during sickness, the body stops converting T4 to T3
When is thyroid antibody testing carried out?
4
- To diagnose autoimmune thyroid disease
- When patient has another type of autoimmune disease and has symptoms suggesting thyroid dysfunction
- To investigate reproductive difficulties
- When woman with autoimmune thyroid disease becomes pregnant (determines risk of thyroid dysfunction in foetus)
What is measuring calcitonin levels used to diagnose?
What are the indicative levels in comparison to the reference range?
C-cell hyperplasia and medullary thyroid cancer
High levels
What is measuring thyroglobulin levels used to diagnose?
What are the indicative levels in comparison to the reference range?
Thyroid cancer
High levels
What further testing should be carried out if TH is normal and TSH is increased?
Thyroid antibodies
What further testing should be carried out if TH is increased and TSH is normal/increased?
Why is this?
TRH test or image pituitary
These levels could indicate TH resistance or a TSH secreting tumour
What are the two broad categories that can be used to classify thyroid dysfunction?
Hyperthyroidism and hypothyroidism
What can cause hyperthyroidism?
5
Graves disease Other autoimmune disease Thyroiditis Thyroid nodules Drugs e.g. lithium
