Is it Possible to Predict T3 Dosage When Changing Thyroid Medication to T3?

These type of questions arises frequently.

  • Should T3 and T4 medication be used in a fixed ratio, or use FT4 and FT3 results to determine if the treatment is correct?
  • Is it possible to pre-determine how much T3 someone might need? This question is frequently expressed as, “If I am on T4 or NDT, and want to switch to T3, can how much T3 is equivalent be calculated?”

Here are my typical responses to these types of questions:

Should T3 and T4 be used in a fixed ratio?

Some people think that because natural desiccated thyroid (NDT) has 4:1 the amount of T4 to T3 that people take always T4 and T3 together in this ratio.

Unfortunately, maintaining a simplistic ratio of T4 to T3 of 4:1 is not going to work for a lot of thyroid patients.

Thousands of people use natural desiccated thyroid (NDT). NDT has a ratio of T4 to T3 of around 4:1. If a 4:1 ratio of T4 to T3 was going to be the right way to go for all people, all of these people on natural desiccated thyroid would get better. They do not though. Some do of course but some need to reduce the NDT and add T3 to it thus reducing the T4:T3 ratio to less than 4:1, i.e. more T3.

Some need to drop NDT altogether and use only T3, i.e. a ratio of 0:1 of T4 to T3.

The fact that the people that increase the ratio of T3 to T4 then get well proves without question that some people need more T3 than a simple 4:1 ratio.

In my own case, I would not be able to function at all with any T4. I know many others like me who have got their lives back by excluding all T4 from their bodies and using only T3.

Other thyroid patients find that NDT has too much T3 content in it and they find that they need to either reduce the NDT and add some T4 or opt for a synthetic T4/T3 combination that allows the ratio of T4 to T3 to be controlled.

A similar argument applies to attempting to use the laboratory test results of FT4 and FT3 to calculate if the treatment is now ideal. Each thyroid patient has a unique system and unique needs in terms of their FT4 and FT3 levels. There is no correct or ideal ratio of FT4:FT3.

Every individual requires a unique solution of T4 and T3 in a ratio that suits their own needs and their own individual issues.

If all doctors insisted on a simplistic 4:1 ratio of T4 to T3,  this would effectively force many thyroid patients to be in permanent ill-health!

This argument is supported by a vast amount of patient experience.

All you can do is to assess how the thyroid patient responds to the thyroid medication. The response of the medication tells us how the body responds at the cellular level. If the thyroid patient remains symptomatic, often the answer is to reduce the T4 content because it can be the T4 itself, or the reverse T3, that is produced from the T4 that is the issue or a marker of the issue.

Bodyweight will also not help the dosing of T3 or T4 greatly, as so much depends on the metabolism of the individual.

We are not robots with exactly the same systems.

Can you predict how much T3 someone might need if they were on a certain dose of T4 before when they switch to T3?

There is no way of assessing what T3 dose an individual thyroid patient might need when they switch from T4 to T3. If they need to switch, the T4 probably is not working well, which makes it harder to get an idea. 

I will say that most thyroid patients do well with T3 dosages in the range of 40 to 80 mcg per day. Some need less and some need more than this.

Endocrinologists say that 40-60 mcg of T3 is a normal, full replacement dose.

In endocrinology books about 20 years ago, they said that 40-70 mcg of T3 was a normal full replacement dose.

However, these assessments do not allow for people who have metabolic issues that stop thyroid hormone working correctly. So, this may be why some thyroid patients need considerably more than 80 mcg of T3.

I do not believe that you can assess how much T4 a thyroid patient is using, and just work mathematically the equivalent of T3 dosage.

What is needed is to provide the right level of T3 that actually corrects symptoms and signs. By slowly increasing the T3 and watching symptoms and signs carefully, it is possible to provide only what the body needs and no more.

Thyroid blood tests and prior assessment of how much T4 they were on will not reveal what a thyroid patient actually needs.

See the Recovering with T3 book for more information on a safe and effective dosing protocol for transitioning to T3.

Best wishes,


(Updated in February 2019)

Paul Robinson

Paul Robinson is a British author and thyroid patient advocate. The focus of his books and work is on helping patients recover from hypothyroidism. Paul has accumulated a wealth of knowledge on thyroid and adrenal dysfunction and their treatment. His three books cover all of this.

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