explain thyroid hormone production before entering thyroid gland
hypothalamus releases TRH
TRH moves to anterior pituitary which releases TSH
TSH moves to thyroid gland to stimulate release of T3 and T4
explain thyroid hormone production once inside the thyroid gland
iodide is pumped into the cell from the blood
moves to colloid (where T3 and T4 are produced)
iodide is oxidised and stuck onto by thyroglobulin tyrosines
this gives MIT - 1 iodide and DIT - 2 iodides
these are coupled to give T3 and T4
T3 = MIT + DIT
T4 = DIT + DIT
lysosomes cut thyroglobulin so there is free T3 and T4 in the blood
explain T3
made from MIT + DIT
active
most comes from deiodination of T4
not soluble in blood so has to be bound to TBG and TTR
explain the secretion of T3 and T4
hypothalamus releases TRH which moves to the anterior pituitary to release TSH
TSH affects thyroid gland to stimulate secretion
when levels of these rise there is a negative feedback loop on the hypothalamus to stop TRH and on the pituitary to stop TSH release
more thyroid hormone = less drive to make more
whats euthoroid state
thyroid hormone secretion is normal
actions of TSH
T3 and T4 production
increased protein synthesis
increased DNA replication
increased cell division
actions of T3 and T4
Thyroid glands - pacemakers of metabolism
increase in t3 –>
- increased metabolic rate
- increased heat
- increased response to sympathetic input
- permits normal growth and development
T4 undergoes deionisation to T3
what is cretinism
iodide deficiency resulting in
- mental retardation
- impaired growth
- deafness
caused by inadequate intake/ maternal iodide deficiency in pregnancy
tests to check thyroid function
TSH (normal = 0.27 –> 4.2)
free T4 (normal = 12–>22 )
free T3 (normal = 3.1 –> 6.8)
signs and symptoms of hypothyroidism
cold intolerance
weight gain
slow HR
tiredness
constipation
forgetfulness
puffy face
pale, dry skin
primary hypothyroidism
low T4 and high TSH
due to damage/ loss of thyroid tissue or inadequate iodide intake
whats the most common cause of hypothyroidism
hashimotos thyroiditis
- damage to cells producing T4 - can result in goitre
whats the treatment for hypothyroidism
levothyroxine - synthetic T4
liothryonine - synthetic T3
side effects of levothyroxine
hair loss
headache
sleep problems
nervousness
fever
increased heart beat
apetite changes
weight loss
secondary hypothyroidism
TSH low, T4 low
pituitary doesn’t produce TSH or hypothalamus doesn’t produce TRH
what is myxodoema coma
end result of untreated hypothyroidism
progressive weakness resulting in unconcioussness
extreme hypothermia
causes -
- illness
- infection
- trauma
- meds that suppress CNS
- exposure to cold
drugs affecting thyroid function
corticosteroids
lithium
amiodarone
signs and symptoms of hyperthyroidism
heat tolerance
weight loss
palpitations
restfulness
fatigue
increased sweating
goitre
hyperthyroidism
thyroid produces excess T4
TSH low, T4 high
most common cause of hyperthyroidism
graves disease
- autoimmune disease caused by thyroid stimulating immunoglobulin
(TSI)
which activates TSH on follicular cells –> increased T4 and T3
treatment for hyperthyroidism
surgery
radioactive iodide to destroy some cells producing thyroid hormones
antithyroid drugs
- carbimazole and propylthioiracil
these inhibit thyroid peroxidase (enzymes responsible for deodiniisation of T4 ) and prevents hormone synthesis
down side of antithyroid drugs (carbimazole and propylthioracil)
increases risk of infection
are thyroid hormones active bound or unbound to proteins
unbound
what converts tyrosine to MIT and DIT
thyroid peroxidases in the thyroid gland
