Levothyroxine: clinical use

Prescribing considerations

• Confirm the diagnosis and assess thyroid function before treatment; use clinical response and biochemical results for subsequent adjustment.
• Identify adrenal insufficiency and provide adequate corticosteroid cover before initiating levothyroxine.
• Use particular caution in older people, long-standing severe hypothyroidism, cardiovascular disease, diabetes and patients at increased osteoporosis risk.
• Levothyroxine is not appropriate for weight reduction in euthyroid people.
• If symptoms arise after a tablet-product switch, assess thyroid function; persistent problems may justify a consistently tolerated product or consideration of oral solution.

Contraindications and cautions

• Hypersensitivity to levothyroxine or product excipients.
• Untreated or inadequately treated adrenal insufficiency.
• Initiation during acute myocardial infarction, acute myocarditis or acute pancarditis for selected oral products; the injectable product additionally contraindicates decompensated cardiac disease.
• Untreated hyperthyroidism or thyrotoxicosis, subject to product-specific wording.
• Levothyroxine–antithyroid combination treatment for hyperthyroidism during pregnancy.
• For the injectable product, untreated pituitary insufficiency when it causes adrenal insufficiency requiring treatment.

Monitoring

• Monitor TSH, with free thyroid hormones where clinically indicated, and review symptoms, adherence and interacting medicines.
• Use closer monitoring during pregnancy, after formulation or product switching, and in cardiovascular disease, thyroid cancer or unstable control.
• Avoid biochemical overtreatment, particularly where atrial fibrillation or fracture risk is increased.
• Consider biotin interference when laboratory results conflict with the clinical picture.

Clinical pharmacology

Levothyroxine is synthetic T4. It is highly protein-bound and converted by tissue deiodination to active T3, which regulates transcription through nuclear thyroid receptors. Its long biological persistence means effects of changes may not be immediate.

Formulation and product differences

• The selected tablet is an uncoated immediate-release product and should not be crushed and dispersed for young children because inaccurate measurement may result.
• Selected oral solutions are supplied with a graduated oral syringe and bottle adaptor; they contain glycerol and a parahydroxybenzoate preservative and require product-specific storage and handling.
• Switching between tablets and oral solution, or between levothyroxine products, may alter thyroid control and warrants clinical and biochemical review.
• The injectable solution is intended for myxoedema coma or when oral treatment is not feasible and is administered by healthcare professionals.

Oral solution

Route: Oral

Strengths: 25 micrograms/5 mL, 50 micrograms/5 mL, 100 micrograms/5 mL, 125 micrograms/5 mL

Tablet

Route: Oral

Strengths: 25 micrograms, 50 micrograms, 100 micrograms