It Isn't Possible to Use Ratios of T4:T3 or Predict T3 Dosage

I was recently asked a couple of questions related to whether T4 and T3 should be used in a fixed ratio and whether it is possible to pre-determine how much T3 someone might need. Here are the questions and my responses.

Question 1:

Could you please share your thoughts about ratios between T4 and T3. Natural desiccated thyroid has 4:1 the amount of T4 to T3 so shouldn't people take T4 and T3 together in this way?


There is a very simple argument that shows that maintaining a simplistic ratio of T4 to T3 of 4:1 is not going to work for all people.

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 were going to be the right way to go for all people then all these people on natural desiccated thyroid would get better. They don't. Many do 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.

Every individual requires a solution of T4 and T3 that suits their own needs and whatever underlying issues they have developed. 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 - not a smart move.

This argument is supported by mountains of patient experiences.

Remember that, when measured at the cell nucleus, FT3 is ten times more potent than FT4. You can't use that information even to assess how much T4 or T3 is needed. All you can do is assess the response of the medication. The response of the medication tells us how the body responds at the cellular level. If someone remains hypothyroid, 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 can be interfering with the action of T3.

Body weight will also not help dosing of T3 or T4 greatly, as so much depends on the individual metabolic problems - we are not robots with exactly the same systems.

Question 2:

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 someone might need when they switch from T4 to T3. If they need to switch, then the T4 probably isn't working well, which makes it harder to get any idea. 

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

Endocrinologists say that 40-60 mcg is a normal full replacement dose. In endocrinology books about 20 years ago they said that 40-70 mcg 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 a lot more than 80 mcg.

I don't think you can look at what someone is taking of T4 and just work out the equivalent of T3 dosage. What is needed is providing the right level of T3 that actually corrects symptoms. 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 won't tell us what a person actually needs.

Best wishes,


(Updated in February 2019)