Why T4 Is First-Line, When T3 Might Be Added, and What to Know About Natural (Whole) Thyroid Extracts
A practical guide to when, why, and how T3 may be considered—based on clinical guidelines and current best practice.

What is Hypothyroidism—and Why Do We Treat It?

Hypothyroidism occurs when the thyroid gland doesn’t produce enough thyroid hormone. These hormones—especially T3 (triiodothyronine) and T4 (thyroxine)—are essential for regulating metabolism, energy, mood, temperature control, and other vital functions. When thyroid hormone is low, the metabolism slows down. Treatment aims to restore healthy hormone levels so you can feel and function at your best

What is Liothyronine (T3)?

Liothyronine is a synthetic form of T3—the active thyroid hormone that works directly on cells throughout the body.

The standard treatment for hypothyroidism in New Zealand is levothyroxine (T4), a longer-acting hormone that your body converts into T3 as needed.

Why is Levothyroxine (T4) Usually Preferred?

Levothyroxine is the first-line treatment for most people with hypothyroidism because it is:

• Stable and long-acting, allowing for once-daily dosing
• Converted into T3 as needed by the body
• Less prone to hormonal fluctuations across the day
• Supported by decades of evidence for safety and effectiveness
• Fully funded in New Zealand

For most people, levothyroxine provides excellent symptom relief and normalises thyroid blood tests.

Finding the Right Dose Takes Time

Levothyroxine doesn’t work instantly. It can take several weeks to months to fully optimise the dose, especially when first starting or during times of physiological change.

Dose adjustments may be needed if:

• You become pregnant (thyroid requirements increase significantly)
• You experience significant weight changes
• You begin new medications that affect absorption
• You’re not taking it consistently on an empty stomach
  (Ideally 30 minutes before food or 2 hours after your evening meal)

What If You Still Don’t Feel Well?

Some people continue to experience symptoms such as fatigue, brain fog, low mood, or fatigue, even when thyroid blood tests are in the normal range on levothyroxine.

Before considering T3, it’s important to rule out other causes:

🧠 Could symptoms be related to iron deficiency, perimenopause, poor sleep, low mood, or other health issues?

In some cases, T3 may appear low, even if TSH and T4 are normal on blood tests. This can reflect reduced peripheral conversion of T4 to T3—common in situations like chronic illness, calorie restriction, stress, ageing, or certain medications. This pattern of blood tests doesn’t always require treatment, but in carefully selected cases, where there are associated symptoms, it may support a discussion around combination therapy.

In short: Is the issue ongoing hypothyroidism—or is there another cause of these symptoms?

When Might T3 Be Considered?

According to the 2015 American Thyroid Association (ATA) Guidelines, T3 may be considered in combination with T4 in specific situations:

✅ Persistent symptoms despite a normal TSH on levothyroxine
✅ After other contributing factors have been ruled out
✅ When the person is well informed and interested in a trial of T3
✅ Under supervision by an experienced clinician

⚠️ Note: Routine use of T3 is not recommended. The evidence is mixed, and not everyone benefits.*

Risks and Considerations

T3 is fast-acting and more potent than T4, which means:

• Blood levels fluctuate more, making stability harder to achieve
• More frequent monitoring is required

Side effects may include:

• Palpitations or irregular heartbeat
• Anxiety or sleep disturbance
• Bone loss with long-term overtreatment

Additionally, T3 is not funded in New Zealand, so patients need to pay for it privately.

How Is T3 Prescribed?

If a decision is made to trial T3:

• It is typically started at a low dose (e.g., 5 mcg once or twice daily)
• It is added to a reduced dose of T4, not used alone
• The dose is adjusted gradually, based on symptoms and blood test results
• Close monitoring is essential

The goal is to improve wellbeing while avoiding overtreatment.

What About  Whole Thyroid/Natural Desiccated Thyroid (NDT)?

Natural desiccated thyroid (NDT) contains both T4 and T3 derived from animal thyroid. Some are drawn to the idea of it because it’s perceived as “natural”—but clinical guidelines advise caution.

Why NDT is not routinely recommended:

• Hormone content can vary between batches, making blood levels harder to stabilise
• Dosing is based on the weight of thyroid tissue, not active hormone content—so the amount of T4 and T3  contained in each dose can vary unpredictably
• T3 levels can spike after each dose, potentially leading to symptoms of overtreatment
• There is no strong evidence showing better outcomes than synthetic options
• Unavailable through standard New Zealand pharmacies and more expensive

For these reasons, synthetic T4 (with or without low-dose T3) remains the safest and most predictable treatment.

Final Thoughts

Most people with hypothyroidism do well on levothyroxine (T4) alone. However, in select cases where symptoms persist despite appropriate treatment and other causes have been excluded, a carefully supervised trial of combination therapy with T3 may be considered.

🧪 These recommendations reflect current best practice as of July 2025. Excitingly, a New Zealand-based study on combination T4/T3 therapy is currently underway at Auckland University. We’ll be watching closely—and look forward to keeping you updated with the latest findings as soon as they’re published.

Sources:

2015 American Thyroid Association Guidelines for the Treatment of Hypothyroidism

Clinical experience and current best practice in endocrinology

Last updated: 28 July 2025