Thyroid physiology Flashcards
Which hormones does the thyroid secrete?
Thyroxine
Tri-iodothyronine
Calcitonin
What number is given to thyroxine?
T4
What number is given to tri-iodothyronine?
T3
Which hormones does the parathyroid gland secrete?
Parathyroid hormone
Which cells within the thyroid are responsible for producing T4 and T3?
Follicle cells
Histologically, how are follicle cells described?
simple cuboidal epithelial
What structure are the follicular cells surrounding?
Colloid
Which cells are scattered around the follicular cells?
Parafollicular C cells
What do parafollicular C cells secrete?
Calcitonin
Which receptors respond to TSH?
Thyrotopin
Describe iodines pathway to the colloid?
Diffuses across the endothelium from capillaries to the follicular cell and diffuses again to the colloid
How many units can DIT link to?
2
By what mechanism does thyroglobulin leave the colloid?
Pinocytosis
How are T3 and T4 released from thyroglobulin?
Enzymatic degeneration by lysosomes which releases thyroglobulin into T3 and T4
Which amino acid does iodine attach to in the follicle?
Tyrosine
What is formed when iodine and tyrosine attach?
MIT
DIT
What is formed when MIT and DIT couple?
T3
What is formed when 2 DITs couple together?
T4
Where are T3 and T4 stored?
In colloid thyroglobulin
Which hormone is the biologically active one?
T3
Where is T4 converted to T3?
Liver and kidney
Which of the hormones are most abundant?
T4 (90%)
What plasma proteins do T3 and T4 bind to?
TBG
TBPA
Albumin
Why do T3/4 bind to plasma proteins?
They are hydrophobic
What happens when T3 and T4 are bound to plasma proteins?
They are inactive.
Does pregnancy increase or decrease T4?
Increases total. No effect on free
Does corticosteroids increase or decrease T4?
Decreases total. No effect on free
Does hepatitis A increase or decrease T4?
Increases total. No effect on free
What effect do anti-epileptics have on T4?
Decrease total. No effect on free
Why does hyperthyroidism cause weight loss?
Increased BMR due to increased number and size of mitochondria therefore increased ATP hydrolysis
Which hormone, important in children, relies on the presence of thyroid hormones?
Growth hormone releasing hormone and therefore growth hormone
Which hormone, used in regulating thyroid hormones, is produced in the hypothalamus?
Thyrotrophin releasing hormone
What does TRH stimulate the release of?
TSH
Where does TSH live?
Anterior pituitary
What does TSH do?
Stimulates release of T3 and T4
How is the negative feedback loop closed?
Presence of T3 and T4
When are thyroid hormones at their highest levels?
Late at night
What causes exopthlamos?
Water retaining carbohydrate build up behind the eyes
Physiologically, why might a hypothyroid be cold?
Thyroid hormones involved in thermogenesis.
When is propanolol used in hyperthryoidism?
To reduce sympathetic effects e.g. increased HR
During pregnancy, what is the effect of the mother being hypothyroid?
Low IQ of baby
Which is of greater significance:
Diurnal rhythm in cortisol or in thyroid hormones?
Cortisol
Which part of the thyroid connects the right and left lobe?
Isthmus
Apart from T3 and T4 what else does the thyroid secrete?
Calcitonin
What is the role of calcitonin?
Regulates calcium
True or False
An underactive thyroid causes heat intolerance
False.
Causes cold intolerance
Which antibodies can interfere with thyroid measurement?
Heterophile antibodies
Why does a raised TSH cause hypothyroidism?
Too much TSH causes negative feedback. No TSH then can’t release T3/4
When is TSH tests not reliable?
If a primary problem with anterior pituitary or hypothalamus
If TSH is suppressed what is the condition present?
Hyperthyroid
