Thyroid hormones Flashcards
What cells make up the thyroid gland?
Follicular cells - simple cuboidal epithelial cells + parafollicular C cells
Which nucleus of the hypothalamus influences thyroid hormone production?
Paraventricular nucleus of the hypothalamus secretes thyroid releasing hormone
Where is TRH released from?
Paraventricular nucleus
What does TRH do?
TRH stimulates the thyrotropes to secrete thyroid stimulating hormone (TSH)
Where is TSH produced from?
Thyrotropes in the anterior pituitary gland
Which hormones does TRH stimulate?
TSH and Prolactin
What kind of hormone is TRH?
A peptide hormone
What causes the synthesis of thyroglobulin?
TSH activation of G protein linked receptors on the follicular cells of the thyroid gland
How is iodide transported into follicular cells of the thyroid?
By iodide/sodium transporters
What is the function of thyroid peroxidase?
Converts iodide to iodine (oxidation of iodide to I2) and then iodinates the tyrosine amino acids of thyroglobulin forming T4 (thyroxine) and T3 (triiodothyronine) = thyroid hormone
What is the alternative name for T4?
Thyroxine
What is thyroxine binding globulin?
A protein secreted by the liver that transports T3 and T4
What is the difference between thyroxine binding globulin and thyroglobulin?
TBG - protein that transports T3 and T4 in blood
Thyroglobulin - a protein formed by thyroid cells under stimulation by TSH, which is iodinised by TPO enzymes to form T3 and T4 hormones
TRH is released by…
The paraventricular nucleus of the hypothalamus
TSH is released by…
Anterior pituitary gland
TSH stimulates…
follicular cells of the thyroid gland to produce thyroglobulin
Thyroperoxidase causes…
oxidation of iodine, iodination of tyrosine amino acids on thyroglobulin and coupling to form T3 and T4
When does thyroid gland development begin in embryology?
By day 24
when is thyroid development complete in embryology?
5-6th week
What are C cells derived from?
4th pharyngeal pouch
Where does the isthmus of the thyroid gland lie?
Anterior to tracheal rings 2-4
What is the main blood supply to the thyroid gland?
Inferior thyroid artery
Where is thyroxine binding globulin made?
Liver
How is thyroid hormone active in cells?
Thyroxine (T4) is deionised at 5 prime carbon to form active T3
What does active T3 do on a molecular level in cells?
Causes the production of sodium/potassium/ATPase pumps, which leads to increased metabolic activity within the cell
What is the impact of thyroid hormone on blood glucose levels?
Hyperglycaemia - They increase by release of glucose from glycogen stores in the liver and stimulate gluconeogenesis
What is the impact of thyroid hormone on the liver?
Glycogenolysis
Gluconeogenesis
Increase uptake of LDL from the blood
What happens to thyroid binding globulins in pregnancy?
They increase
what happens to total T4 and T3 levels in pregnancy?
They increase due to an increase in thyroid binding globulins, therefore free T3 and T4 must be measured
What are TSH levels in pregnancy compared to TSH outside of pregnancy?
Lower in pregnancy
How do free T3 and T4 levels change during pregnancy?
They drop
What happens to TSH levels in pregnancy?
They decrease
Why do TSH levels reduce in pregnancy?
HCG levels increase (HCG is partially thyrotrophic)
In hyperemesis, what is the thyroid function usually like?
High free T3 and T4, low TSH
This is thought to be related to the thyrotropic properties of Beta HCG
What is the effect of Beta HCG on thyroid function?
Beta HCG is thyrotropic and therefore raises thyroid hormone levels in early pregnancy causing a fall in TSH
