Liothyronine (T3) tablets: what are they and who needs them?

If you have an underactive thyroid (hypothyroidism), you are almost certainly on levothyroxine, also called thyroxine or T4. Some people come across another thyroid medicine, liothyronine, also known as T3, and wonder whether it would make them feel better. This page explains what T3 is, when endocrinologists use it, and why it is not for everyone.

What is the difference between T4 and T3?

Your thyroid gland makes two main hormones:

• T4 (thyroxine) is made in larger amounts. It is fairly inactive on its own and acts as a storage form.
• T3 (triiodothyronine) is the active hormone that actually works inside your cells.

Most of your T3 does not come straight from the thyroid. Your body converts T4 into T3 in the liver, muscles, brain and other tissues, as and when they need it. This is a clever system, because each tissue can adjust its own supply.

Levothyroxine tablets replace T4, and your body makes T3 from it. Liothyronine tablets give T3 directly. In India, liothyronine is available under brand names such as Linorma T3.

How does T3 behave in the body?

T3 is quite different from T4 as a medicine:

• It acts fast. Effects start within hours and reach their full level within a few days. T4 takes several weeks.
• It leaves fast. T3 lasts about a day in the body, while T4 lasts about a week. So T3 is usually taken more than once a day, as prescribed.
• It is strong. T3 is several times more potent than T4, weight for weight. A small difference in strength makes a big difference in effect.
• Blood levels go up and down over the day, which makes test results harder to interpret.

Why do most people only need T4?

For the large majority of people with hypothyroidism, levothyroxine alone works well. It is cheap, it is taken once a day and the body’s own conversion system does the rest. Major thyroid societies, including the American Thyroid Association, recommend levothyroxine alone as the standard treatment.

If your TSH is in range on levothyroxine and you feel well, there is no reason to add T3. If you are unsure whether your thyroid is in range, the thyroid report reader explains common patterns, and the guide to the thyroid function test explains each value.

When might T3 be considered?

1. Persistent symptoms despite normal tests. A minority of people on levothyroxine, roughly 1 in 10, continue to feel tired, foggy or low even when their TSH is normal. Before T3 is considered, doctors first look for other causes, because many conditions can cause these symptoms, such as anaemia, low vitamin B12 or D, poor sleep, sleep apnoea, depression, diabetes or other hormone problems. Also checked is whether the levothyroxine is being absorbed properly, since tea, coffee, calcium and iron near the tablet can reduce absorption.

If no other cause is found, European and American experts say a carefully monitored trial of T4 and T3 together can be considered in selected adults. The European Thyroid Association has called this an experimental approach, not routine treatment. Large studies have not shown clear benefit for everyone, although some individuals do report feeling better. If there is no clear improvement after a few months, the trial is usually stopped.

2. Myxoedema coma. This is a rare, life-threatening state of severe untreated hypothyroidism with drowsiness, low body temperature and slow breathing. It is treated in hospital, and T3 may be part of that treatment.

3. Thyroid cancer care. Some people being prepared for radioactive iodine treatment or scans are given T3 for a short period, because it wears off quickly.

4. Other specialist situations, decided case by case by an endocrinologist.

Is T3 safe in pregnancy?

Generally no, it is avoided. This is an important correction to a common belief.

During pregnancy, especially in the first few months, the baby’s brain needs T4 from the mother. The baby’s own tissues convert this T4 into T3 exactly where it is needed. T3 from tablets does not cross the placenta well, so if a mother is treated with T3, her blood tests can look normal while the baby’s brain may receive too little thyroid hormone.

For this reason, the American Thyroid Association recommends levothyroxine alone for hypothyroidism in pregnancy, and advises against T3 or combinations. If you are taking T3 and are pregnant or planning to be, talk to your doctor promptly so your treatment can be reviewed. Do not change your tablets on your own.

What are the risks of T3?

Because T3 is strong and fast, too much of it can act like an overactive thyroid:

• Palpitations or a racing heartbeat
• Irregular heart rhythm such as atrial fibrillation, especially in older people
• Anxiety, tremor, sweating and poor sleep
• Over many years, possible loss of bone strength

Short-term studies of carefully monitored combination therapy have not shown a clear increase in heart problems or fractures, but long-term data are limited. Doctors are especially cautious in people over 60, those with heart disease and those with osteoporosis.

How is T3 treatment monitored?

Your doctor will check TSH and often free T4 and free T3. With T3, the timing of the blood test relative to your tablet matters, because levels rise and fall over the day. Your doctor will tell you how to time the test. TSH may run lower than usual on T3, and your doctor will interpret this in context.

What should you ask your doctor?

• Are my thyroid levels already in the right range on levothyroxine?
• Have other causes of my tiredness been checked?
• If we try T3, how will we judge whether it is helping, and for how long?
• Is T3 safe for me, given my heart, bones and pregnancy plans?

Hypothyroidism can show up in many ways, including hair fall; read whether your hair loss is due to your thyroid. Decisions about T3 are best made with an endocrinologist; see who an endocrinologist is or book a consultation.