Levothyroxine (T4) — NZ Medication Guide

ByRamon Wong, BPharm

✅ Reviewed by Ramon Wong, BPharm, Registered Pharmacist (New Zealand) | May 2026
This information is for educational purposes only and is not a substitute for personalised advice from your doctor or pharmacist. Always seek professional guidance before making changes to your medications.

🏷️ NZ brand names: Eltroxin (most common), Synthroid, Levothyroxine (BOU generic)

What is Levothyroxine (T4)?

Levothyroxine (thyroxine, T4) is a synthetic thyroid hormone used to treat hypothyroidism and as suppression therapy in thyroid cancer. This medication is funded by Pharmac for hypothyroidism and thyroid cancer.

What is Levothyroxine (T4) Used For?

Levothyroxine is prescribed for hypothyroidism (underactive thyroid), thyroid hormone replacement after thyroidectomy, Hashimoto’s thyroiditis, and as TSH suppression therapy in differentiated thyroid cancer.

How Does Levothyroxine (T4) Work?

Levothyroxine is identical to the T4 thyroid hormone naturally produced by the thyroid gland. It is converted in peripheral tissues to the active form triiodothyronine (T3), which regulates metabolism, cardiac function, neurological development, and the function of virtually every organ system.

How to Take Levothyroxine (T4)

Taken once daily in the morning on an empty stomach, 30–60 minutes before breakfast, with a full glass of water. Do not take within 4 hours of calcium supplements, iron, antacids, or certain other medicines that impair absorption. TSH is monitored every 6–8 weeks until levels are stable, then annually.

Common Side Effects of Levothyroxine (T4)

  • Usually none at the correct dose
  • Symptoms of over-replacement: palpitations, tremor, sweating, weight loss, anxiety, insomnia
  • Symptoms of under-replacement: fatigue, weight gain, cold intolerance

Serious Side Effects — Seek Medical Attention

Contact your doctor or call 111 immediately if you experience any of the following:

  • Cardiac arrhythmias (atrial fibrillation) — risk with over-replacement, especially in elderly patients or those with cardiac disease
  • Osteoporosis (long-term TSH suppression — particularly in postmenopausal women)
  • Adrenal crisis (if undiagnosed adrenal insufficiency — always check cortisol before starting levothyroxine in suspected pituitary/adrenal disease)

Drug Interactions

Levothyroxine (T4) may interact with other medicines. Always inform your doctor and pharmacist of all medications you are taking. Key interactions include:

  • Calcium carbonate and antacids (reduce absorption — take 4 hours apart)
  • Iron supplements (reduce absorption — take 4 hours apart)
  • Cholestyramine and colestipol (bind levothyroxine)
  • Warfarin (levothyroxine enhances anticoagulant effect)
  • Oestrogens (increase thyroid-binding globulin — may need higher levothyroxine dose)

New Zealand Prescribing Information

Levothyroxine (Eltroxin® 50 mcg and 100 mcg tablets; also 25 mcg, 75 mcg, 150 mcg generics) is funded by Pharmac. Consistent use of the same brand is recommended as small differences in bioavailability between brands can affect TSH control — if you change brands, recheck TSH in 6–8 weeks.

Frequently Asked Questions

Why must I take levothyroxine on an empty stomach?

Food, coffee, and many medicines reduce levothyroxine absorption. Taking it 30–60 minutes before breakfast on an empty stomach ensures consistent, maximum absorption and stable thyroid levels.

How will I know if my dose needs adjusting?

Your doctor monitors TSH (thyroid-stimulating hormone) levels in your blood. A high TSH means you may be under-treated; a low TSH means potentially over-treated. The target range depends on your age and whether you have thyroid cancer.

⚠️ Disclaimer: This page is for educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your doctor or a qualified health provider. In New Zealand, medication availability and funding may vary — check with Pharmac or your pharmacist for current information.

Reviewed by a Registered Pharmacist NZ

References & Further Information

The following New Zealand and international resources were used to inform this page:

Related Medications

What is Levothyroxine used for?

Levothyroxine (also known as thyroxine or T4) is used to treat hypothyroidism (underactive thyroid). It replaces or supplements the thyroid hormone that your thyroid gland cannot produce sufficiently.

Is Levothyroxine funded in New Zealand?

Yes, Levothyroxine is funded by Pharmac in New Zealand for patients with confirmed hypothyroidism. It requires a prescription from a doctor.

When is the best time to take Levothyroxine?

Levothyroxine is best taken in the morning on an empty stomach, 30 to 60 minutes before breakfast. This helps the body absorb the medicine properly. Many foods, drinks (including coffee), and other medicines can interfere with its absorption.

What are the symptoms of taking too much Levothyroxine?

Symptoms of too much Levothyroxine include rapid or irregular heartbeat, anxiety, tremors, sweating, weight loss, and difficulty sleeping. Regular blood tests (TSH levels) are important to ensure your dose is correct.

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