Physiology Of The Thyroid Flashcards
Describe the hypothalamic-pituitary-thyroid axis
- Hypothalamus produces TRH
- TRH stimulates anterior pituitary to produce TSH
- TSH binds to TSH GPCR on surface of thyroid epithelial cells, T3 & T4 released
- Negative feedback loop!
Describe the basic structure of the thyroid
Follicles - have an inner colloid that is lined by cuboidal epithelial cells & a basement membrane
Parafollicular (C) cells - in between the base membrane, secrete calcitonin
Where is thyroglobulin synthesised
Follicular (epithelial) cells
Where is thyroglobulin stored
The colloid
What happens to iodide once it is taken up by the epithelial follicular cells
- In epithelial cells - iodide is oxidised to iodine
- In colloid - iodine attaches to tyrosine residues on thyroglobulin to form DIT & MIT
What makes up T3 vs T4
T3 - DIT & MIT
T4 - DIT & DIT
Briefly summarise the formation of T3 & T4 in the colloid upon stimulation by TSH
Iodine attaches to tyrosine residues on thyroglobulin
This forms DIT & MIT, which are the building blocks of T3/4
Where does iodine attach on thyroglobulin
Tyrosine residues
Is T3 or T4 biologically active
T3
Where is T4 activated i.e. converted to T3
Kidneys & liver
How is most of thyroid hormones transported in the blood
Thyroxine binding globulin (70%)
When considering a patients metabolic state, would it be more helpful to test for free thyroid hormone levels or the total thyroid hormone concentration (including those bound to proteins). Why?
Free thyroid hormone levels
- protein bound hormones cant enter cells & have an effect
- throxine bound globulin levels are altered in different conditions
Altered TBG levels can affect the total T4 levels. What would increase TBG?
Pregnancy and oral contraceptive pill
Chronic active hepatitis and biliary cirrhosis
Altered TBG levels can affect the total T4 levels. What would decrease TBG?
Androgens
Cushing’s
Severe systemic illness
Chronic liver disease
What effect does thyroid have on the liver
Increased glycogenolysis and gluconeogenesis
Decreased glycogenesis
I.e. increased glucose use & decreased glucose storage
What effect does thyroid have on the adipose tissue
- Increased lipolysis
- Decreased lipogenesis
I.e. fat burning
What effect does thyroid have on the lungs & heart
Increased RR & HR & contraction
=> increased O2 for metabolism
What effect does thyroid have on metabolic rate & thermogenesis
- Increased BMR
(increased mitochondria & aerobic metabolism)
(increased protein, lipid & carbs metabolism) - Increased thermogenesis
Summarise the effects of thyroid
Increased BMR
- Increased HR & RR => increased O2
- Increased glucose use & decreased storage
- Increased lipid, carbs & protein metabolism (use)
What is the relationship between thyroid hormones & growth hormone
GHRH production & secretion requires thyroid hormones
GH activity requires thyroid hormones
What is the importance of thyroid hormone in the development of foetal & neonatal brain
Thyroid hormone required for myelinogenesis & atonal growth
What is the relationship between thyroid hormones & adrenaline/noradrenaline
Thyroid hormones increase responsiveness of adrenaline and noradrenaline by increasing the number of receptors
What enzymes are involved in the degradation of thyroid hormones
De-iodinases