THYROX

What it is

THYROX is a levothyroxine sodium tablet. The medicine is available in five strengths: 25 micrograms (orange tablets), 50 micrograms (white tablets), 75 micrograms (violet tablets), 100 micrograms (yellow tablets), and 200 micrograms (pink tablets). THYROX is bioequivalent to OROXINE and EUTROXSIG, but is not bioequivalent on a same-dose basis with ELTROXIN. If a decision is made to switch a patient from ELTROXIN to THYROX, prescribers should have a plan for monitoring TSH and should be aware that dose adjustment may be required.

Approved indications

— Management of demonstrated thyroid hormone deficiency. — Suppression of thyrotropin (TSH) for the management of TSH-responsive tumours of the thyroid.

Dosing overview

THYROX is best taken as a single daily dose first thing in the morning, with water on an empty stomach, and at least 30 minutes and preferably 60 minutes before the intake of any food or other medications. The dose should be individualised on the basis of clinical response and biochemical tests, with regular monitoring of TSH and levothyroxine recommended when starting therapy or changing the dose. Adults should commence with levothyroxine 50 to 100 micrograms daily, with the daily dose increased by 25 to 50 micrograms according to response at not less than 4-weekly intervals, up to 100 to 200 micrograms daily. In patients aged 60 years and over and in those with ischaemic heart disease, levothyroxine therapy should normally be initiated with low doses (25 or 50 micrograms per day). Adult maintenance dose is 100 to 200 micrograms per day. Children with congenital and acquired hypothyroidism require approximately 8 micrograms per kilogram per day (0–6 months), 6 micrograms per kilogram per day (6–12 months), 5 micrograms per kilogram per day (1–5 years), 4 micrograms per kilogram per day (6–12 years), and 2 micrograms per kilogram per day (12 years and over).

Key safety warnings

Extreme caution is required in patients with a cardiovascular disorder; in the event of cardiovascular effects, the dosage of THYROX should be lowered, and even smaller initial dosage (12.5 to 25 micrograms per day) should be used with increments of not more than 25 micrograms per day at not less than two-week intervals. Corticosteroid replacement therapy must precede initiation of THYROX therapy to avoid Addisonian crisis in conditions such as hypopituitarism and adrenal insufficiency. In women, long-term levothyroxine sodium therapy has been associated with increased bone resorption and decreased bone mineral density, especially in post-menopausal women on greater than replacement doses or receiving suppressive doses; patients receiving levothyroxine sodium should be given the minimum dose necessary to achieve the desired clinical and biochemical response. Caution is required when levothyroxine is given to hypothyroid patients with diabetes mellitus or diabetes insipidus, as it may increase the required dosage of insulin and oral antidiabetic agents; careful monitoring of diabetic control is recommended, especially when THYROX therapy is initiated, changed or discontinued. Levothyroxine should not be used for the treatment of obesity or weight loss; in euthyroid patients, doses within the range of daily hormonal requirements are ineffective for weight reduction, and larger doses may produce serious or even life-threatening manifestations of toxicity, particularly when given with sympathomimetic amines.

Contraindications

THYROX is contraindicated in patients with known hypersensitivity to thyroxine, untreated hyperthyroidism, uncorrected primary or secondary adrenal insufficiency, thyrotoxicosis, acute myocardial infarction uncomplicated by hypothyroidism, acute myocarditis, and acute pancarditis.

Regulatory history

THYROX was first registered on the Australian Register of Therapeutic Goods on 2 June 2023, with registrations for all five tablet strengths (25, 50, 75, 100, and 200 micrograms).