hypothyroidism

Question 1

describe how T3 and T4 are made

Answer 1

• iodidine ions in presence of TPO and hydrogen peroxide are converetd to a reactive iodine form
• I* then iodinates one (MIT) or two (DIT) positions on TG to create mono-iodotyrosines (MIT) or di-iodotyrosines (DIT)
• TPO and hydrogen peroxide catalyse a coupling reaction to create T3 or T4
• lysosomes uptake clumps of colloid when is broken down to liberate T3 and T4
• moves into blood

Question 2

what is primary hypothyroidism?

Answer 2

• myxoedema
• AI damage to thyroid
• T4 levels low, TSH levels high

Question 3

what are the symptoms?

Answer 3

• deepening voice
• depression and tiredness
• cold intolerance
• weight gain
• constipation
• bradycardia
• eventual myxoedema coma

Question 4

how is T4 converted into T3?

Answer 4

deiodinase

Question 5

what is circulating T3 made up of?

Answer 5

• 80% from de-iodination of T4

- 20% from direct thyroidal secretion

Question 6

how does T3 have its effects?

Answer 6

• T3 travels to nucleus
• binds to a heterodimer of thyroid hormone receptor and retinoid x receptor
• binds onto the DNA parts called the thyroid response element
• effects

Question 7

what is used for thyroid hormone replacement therapy?

Answer 7

levothyroxine sodium

Question 8

what 3 scenarios is levothyroxine sodium used?

Answer 8

• AI primary hypothyroidism
• iatrogenic primary hypothyroidism
• secondary hypothyroidism

Question 9

when do you give T3?

Answer 9

when want a rapid effect
e.g. in myxoedema coma
given IV

Question 10

why are some people given a combo of T3 and T4?

Answer 10

some patients reported improvement in well-being when given combo

Question 11

what is the issue of T3?

Answer 11

can get toxicity effect

• palpitations
• tremors
• anxiety

Question 12

what are the adverse effects of thyroid hormone over replacement?

Answer 12

• Skeletal: inc. bone turnover, reduction in bone density
• Cardiac: tachycardia, risk of dysrhythmia, atrial fibrillation
• Metabolism: inc. energy expenditure, weight loss
• Inc. beta-adrenergic sensitivity: tremor, nervousness

Question 13

what are the half lives of T3 and T4?

Answer 13

T4 - 6 days

T3 - 2.5 days

Question 14

how much T3 and T4 are plasma protein bound? which plasma proteins?

Answer 14

99.97% of T4
99.7% of T3
mainly TBG

Question 15

when do PPBs inc?

Answer 15

• pregnancy

- during prolonged treatment with oestrogens and phenothiazines

Question 16

when do TBGs fall?

Answer 16

• malnutrition
• liver disease
• certain drug treatments

Question 17

how is free/unconjugated hormone secreted?

Answer 17

in bile and urine
T3 cleared in hours
T4 cleared in about 6 days