17 The thyroid

Question 1

17.01 THYROID HORMONE SUPPRESSION

Carbimazole, propylthiouracil - actions

Answer 1

gradually decreases thyroid hormone output and thus reduces signs and symptoms of thyrotoxicosis

Question 2

17.01 THYROID HORMONE SUPPRESSION

Carbimazole, propylthiouracil - MOA

Answer 2

reduces the synthesis of thyroid hormones by inhibiting thyroperoxidase which normally iodinates tyrosyl residues in thyroglobulin to give the precursors of T3 and T4

Question 3

17.01 THYROID HORMONE SUPPRESSION

Carbimazole, propylthiouracil - abs/distrib/elim

Answer 3

given orally
carbimazole is converted to methimazole, plasma half-life 6-15h

Question 4

17.01 THYROID HORMONE SUPPRESSION

Carbimazole, propylthiouracil - clinical use

Answer 4

hyperthyroidism
the clinical response may take several weeks because the thyroid stores of hormone need to be depleted and T4 has a long half-life

Question 5

17.01 THYROID HORMONE SUPPRESSION

Carbimazole, propylthiouracil - adverse effects

Answer 5

agranulocytosis (rare - incidence 0.1–1.2%)
rashes (more common)
joint pains

Question 6

17.02 THYROID HORMONE REPLACEMENT

Levothyroxine (synthetic T4); similar: liothyronine (T3) - actions

Answer 6

increased metabolism of carbohydrates, proteins and fats
increase in basal metabolic rate

Question 7

17.02 THYROID HORMONE REPLACEMENT

Levothyroxine (synthetic T4); similar: liothyronine (T3) - MOA

Answer 7

the drug enters cells and is converted to T3, which enters the nucleus and binds to a thyroid hormone receptor
the complex activates transcription, resulting in the generation of mRNA and the synthesis of proteins and enzymes responsible for the metabolic actions of T4

Question 8

17.02 THYROID HORMONE REPLACEMENT

Levothyroxine (synthetic T4); similar: liothyronine (T3) - abs/distrib/elim

Answer 8

given orally
long half-life
an IV formulation of liothyronine is available

Question 9

17.02 THYROID HORMONE REPLACEMENT

Levothyroxine (synthetic T4); similar: liothyronine (T3) - clinical use

Answer 9

hypothyroidism
liothyronine injection is used for myxoedema coma

Question 10

17.02 THYROID HORMONE REPLACEMENT

Levothyroxine (synthetic T4); similar: liothyronine (T3) - adverse effects

Answer 10

nervousness, palpitations, insomnia, heat intolerance, weight loss

Question 11

17.02 THYROID HORMONE REPLACEMENT

Levothyroxine (synthetic T4); similar: liothyronine (T3) - special points

Answer 11

best given on an empty stomach since some foods can interfere with absorption