Levothyroxine

Question 1

What is levothyroxine?

Answer 1

• major hormien derived from thyroid gland

Question 2

What class of compounds does levothyroxine belong to?

Answer 2

thyroid hormones

Question 3

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

Answer 3

Iodine

liothyronine

Question 4

What is levothyroxine indicated for?

Answer 4

hypothyroidism

Question 5

What is the mechanism of levothyroxine?

Answer 5

• 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.

Question 6

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

Answer 6

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

Question 7

What are the precautions associated with levothyroxine?

Answer 7

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

Question 8

What are the contraindications with levothyroxine?

Answer 8

• thyrotoxicosis
• acute MI
• uncorrected adrenal insufficiency

Question 9

What are the adverse effects with levothyroxine use?

Answer 9

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

Question 10

What are the relevant drug interactions with levothyroxine use?

Answer 10

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

Question 11

What are the alarm bells with levothyroxine use?

Answer 11

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