Physiology of thyroid and HPT axis

Question 1

What is an enlarged thyroid gland called?

Answer 1

goitre

Question 2

Histology of thyroid

Answer 2

made of follicles
height of follicular cells is dependent on state of activation by TSH

colloid = thyroid gland’s store of thyroid hormone

parafollicular cells that make calcitonin are between the follicles

Question 3

What is the biologically active thyroid hormone?

Answer 3

T3

Question 4

What causes Graves exophthalmos?

Answer 4

cross reaction of thyroid stimulating antibody in Graves against the TSH receptor on orbital tissue

Question 5

What skin condition can be seen in Graves disease? What causes it?

Answer 5

pre-tibial myxoedema
cross reaction of thyroid-stimulating antibodies against the skin

Question 6

What are the steps in thyroid hormone biogenesis?

Answer 6

Iodide uptake by sodium iodide importer

Thyroglobulin synthesis

Hydrogen peroxide generation

Organification - incorporation of iodine into thyroglobulin - done by TPO

Coupling - 2xDIT brought together (makes T4) or MIT and DIT brought together (makes T3) - done by TPO - stored in colloid

Uptake and breakdown of colloid - cathepsins in lysosome breakdown thyroglobulin releasing T3 and T4 into circulation

Free MIT/DIT is deiodinated by iodotyrosine dehalogenase so the iodine can be recycled

Question 7

How is most thyroid hormone transported?

Answer 7

bound to proteins

small amount is free

Question 8

Diagnosis:
Low T4, Low T3, High TSH

Answer 8

primary hypothyroidism

Question 9

Diagnosis:
High T4, High T3, Low TSH

Answer 9

primary hyperthyroidism

Question 10

Diagnosis:
Low T4, Low T3, Low TSH

Answer 10

secondary/tertiary hypothyroidism

secondary = pituitary isn’t making TSH
tertiary = hypothalamus isn’t making TRH

Question 11

Diagnosis:
High T4, High T3, High TSH

Answer 11

secondary/tertiary hyperthyroidism

Question 12

Diagnosis:
Low/Normal T4, Low T3, Normal TSH

Answer 12

sick euthyroid syndrome
response to stress
shouldn’t measure thyroid hormones when patient is unwell (unless hypo/hyperthyroidism is thought to be the cause), wait 6-8 weeks

Question 13

What foods provide iodine?

Answer 13

fish
seaweed
shrimp
dairy (milk, yoghurt and cheese)
iodised salt (available in US and other countries)

Question 14

How is iodine excreted?

Answer 14

urine and faeces

Question 15

Which hormone stimulates the thyroid gland to make thyroid hormones?

Answer 15

thyroid stimulating hormone (TSH)

Question 16

TSH structure

Answer 16

glycoprotein
alpha and beta subunits