Levothyroxine Flashcards

1
Q

What is levothyroxine?

A
  • major hormien derived from thyroid gland
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2
Q

What class of compounds does levothyroxine belong to?

A

thyroid hormones

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3
Q

What are examples of other drugs in the same class as levothyroxine?

A

Iodine

liothyronine

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4
Q

What is levothyroxine indicated for?

A

hypothyroidism

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5
Q

What is the mechanism of levothyroxine?

A
  • acts like endogenous thyroxine to relieve hypothyroidism
  • T4 is converted to T3 (active) in the kidneys. They attach to thyroxin-binding globulin and thyroixin-binding prealbumin to increase solubility.

they are then transported to receptors in the cytoplasm and nucleus.

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6
Q

What are the relevant PD/PK parameters with levothyroxine use?

A

48-80% bioavailable
more than 99% bound to serum proteins
approx 70% of secreted T4 is deiondinated to equal amounts of T3 and reverse T3.

primarily eliminated metabolites.

half life is 6-7 days for T4 and 1-2 days for T3

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7
Q

What are the precautions associated with levothyroxine?

A

-panhypoituitarism, predisposition to adrenal insufficiency.
elderly,
cardiovascular disorders

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8
Q

What are the contraindications with levothyroxine?

A
  • thyrotoxicosis
  • acute MI
  • uncorrected adrenal insufficiency
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9
Q

What are the adverse effects with levothyroxine use?

A

usually due to excessive dosage;

  • diarrhoea,
  • increased appetite
  • vomiting
  • anginal pain
  • arrhythmias
  • palpitations
  • tachycardia
  • hypertension
  • heart failure
  • MI
  • dyspnoea
  • tremor
  • excitability
  • insomnia, headache
  • flushing, sweating, fever, heat intolerance
  • menstural irregularities, impaired fertility,
  • weight loss, muscle cramp, muscular weakness, transient hair loss

-hypersensitivity reactions

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10
Q

What are the relevant drug interactions with levothyroxine use?

A

efficacy is reduced by calcium carbonate, colsevelam, colestilan, colestyramine need to separate by at least 4 hours

-hypothyroidism symptoms when taken with orlistat
-hypothyroid reduces theophylline metabolism while hyper increases it.
-may develop elevated levels of TSH when given with amiodarone
OCs appear to increase requirements of levothyroxine

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11
Q

What are the alarm bells with levothyroxine use?

A

Baeline ECG valuable as changed induced by hypothyroidism can be confused with ischaemia.
if metabolism increases too rapidly, reduce dose/withold for 1-2 days and start at lower dose

although pregnancy class A, levothyroxine may cross placenta
excessive/insufficient maternal thyroid hormones detrimental to fetus
levothyroxine requirement may increase during pregnancy

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