Levothyroxine — Thyroid Medication Guide | KiwiMeds
✅ Reviewed by a Registered Pharmacist NZ | Last updated: May 2026 | This information is for educational purposes only and does not replace advice from your doctor or pharmacist.
What is levothyroxine?
Levothyroxine (also called L-thyroxine or T4) is a synthetic form of the hormone thyroxine, which is normally produced by your thyroid gland. It is the standard treatment for hypothyroidism (underactive thyroid) in New Zealand. Brand names include Eltroxin and Thyronorm; generic levothyroxine is also available.
What is it used for? (NZ context)
Levothyroxine is prescribed for hypothyroidism (when your thyroid gland doesn’t produce enough thyroid hormone), after thyroid removal surgery, as part of thyroid cancer treatment, and for some cases of thyroid nodules (goitre). It is funded by Pharmac.
How does it work?
Levothyroxine replaces or supplements the thyroxine that your thyroid gland is not producing in adequate amounts. Thyroxine affects nearly every organ in your body — it regulates your metabolism, heart rate, body temperature, digestion, muscle function, brain development, and bone maintenance. Taking levothyroxine restores normal thyroid hormone levels.
How to take it
Always take levothyroxine exactly as directed by your doctor or pharmacist. Do not stop or change your dose without medical advice.
Common side effects
- Usually very few side effects when the dose is correct
- If slightly over-dosed: heart palpitations, anxiety, tremor, insomnia, weight loss, diarrhoea, heat intolerance
- If slightly under-dosed: continued hypothyroidism symptoms — fatigue, weight gain, constipation, cold intolerance, depression
Serious side effects to watch for
The main risk is taking the wrong dose. Too much levothyroxine (over a long period) can increase the risk of osteoporosis (bone thinning) and atrial fibrillation (irregular heart rhythm). Too little means your hypothyroidism is not adequately treated. Regular thyroid function blood tests (TSH level) are essential.
Important drug interactions
- Calcium supplements, antacids, iron supplements — can reduce levothyroxine absorption; take at least 4 hours apart
- Cholestyramine and some cholesterol medicines — reduce absorption
- Warfarin — levothyroxine may enhance its effect
- Diabetes medicines — thyroid levels affect blood sugar control; doses may need adjusting when levothyroxine dose changes
- Certain antidepressants — interactions possible; discuss with your doctor
NZ-specific information
Pharmac funding: Levothyroxine is fully funded. Available as Eltroxin tablets (25mcg, 50mcg, 100mcg) and generic levothyroxine. Brand consistency: It is recommended to remain on the same brand of levothyroxine consistently, as small differences between brands can affect thyroid levels. If your brand changes, ask for your TSH to be checked. Testing: TSH blood tests should be done 6-8 weeks after starting or changing dose, and then annually once stable.
Frequently asked questions
How should I take levothyroxine? Take it on an empty stomach, 30-60 minutes before breakfast, with a glass of water. Many medicines and foods (especially calcium, iron, coffee, soy) can reduce its absorption — so the morning, pre-breakfast routine is important. Will I be on it forever? Most people with hypothyroidism need levothyroxine lifelong. Stopping it allows hypothyroidism to return.
Related medications
Metformin (diabetes) | Atorvastatin (cholesterol) | Sertraline (depression/anxiety)
Always talk to your pharmacist or doctor for personalised advice. This information is general and educational — it does not replace a consultation with your healthcare provider.
References & Further Information
The following New Zealand and international resources were used to inform this page: