Ideal FT3/FT4 ratio/T4:T3 Ratio

[Kriticat]

What are the ideal ratios? I know I've read something about it recently but I've been searching and I can't find it again!

[SeasideSusie Remembering]

Kriticat

If a patient is on levo only it's possible to work out how well they convert T4 to T3. You do this by dividing the FT4 result by the FT3 result providing the unit of measurement is the same for both.

Good conversion takes place when the ratio of T4 : T3 is 4:1 or less. For example

FT4: 20

FT3: 5.5

20÷5.5 = 3.63 so that's good conversion

FT4: 20

FT3: 4

20÷4 = 5 so that's poor conversion

[Lalatoot]

If you are looking for ideal blood results to aim for then the ideal ratio is one where you personally feel well. Don't aim to mimic someone else's ideal or try to recreate results from elsewhere. Your body needs what it needs.When folks on here say they for example need ft4 70% through range and ft3 65%through range to feel well that's just it for them and it may not be right for you . My ft4 is 50%and my ft3 72% and that works for me.

[radd]

Kriticat,

If you refer to a supposed ratio of T4:T3 we need to recover wellbeing, we all have individualised and  variable ratios because of dependancy upon secretion, conversion and multifactorial environmental/health issues, even antibodies and nodules will influence. This is evidenced by two people medicating the same ratio of combo meds who have different lab ratios as hormones are utilised differently.

In a healthy thyroid T3:T4 ratios are flexible enough to allow a steady level of T3 where as when we medicate (often 100% exogenous hormone) we lose a proportion of flexibility and observation of the T3 levesl becomes more important than T3 ratio which if used to indicate med doses may risk too little or too high FT3 levels.

SeasideSuzie has supplied a way of calculating our conversion ratio observed in labs when medicating Levo mono therapy but remember this ratio may not be where you as an individual function best, and serves only as an indicator that a proportion of T4 is indeed being converted. A good example of these irregularities is shown when a tiny bit of T3 is medicated alongside T4 and T3 levels raise significantly higher than that tiny amount medicated because of the effect on the conversion enzymes (deidinases). 

Tania Smith provides an interesting blog on T4:T3 ratios here ..... thyroidpatients.ca/2019/05/...

[Lalatoot in reply to radd]

Excellent explanation of why some of us need only a little lio added to levo. Thanks radd . Koola relevant to our discussions and the link makes for an informative read.

[jimh111]

The fT3:fT4 ratio has very limited use. In healthy people this ratio varies as the body responds to TSH and fT4 to preserve fT3 levels.

Assume typical reference intervals. TSH (0.27 - 4.2), fT3 (3.5 - 6.5), fT4 (12.0 - 22.0). Then

TSH 0.8, fT3 5.0, fT4 19.0 and

TSH 2.4, fT3 5.0, fT4 14.0 are equally good results with very different fT3:fT4 ratios.

A damage thyroid can give very misleading results, as it fails TSH rises and the thyroid secretes proportionally more T3 and less T4 making the ratio 'good'. The same can happen with iodine deficiency.

More complex is the role of the deiodinases. When hormone levels fall type-2 deiodinase activity (D2) increases. D2 is active close to the cell nucleus in tissues such as the brain and skeletal muscles and so T3 originating from D2 reflects cellular T3 to an extent. Type-1 deiodinase (D1) increases as hormone levels increase. D1 takes place near the cell membrane, D1 supplies circulating T3 (and rT3). Thus,

1. fT3=5, fT4=17.5

2. fT3=6, fT4=21

would give the same fT3:fT4 ratios but reflect very different thyroid statuses. The person in case 1 will likely be in a better place as regards their thyroid status.

A further (minor) complacation is people with the DIO2 rs225014 polymorphism, they will have a slightly lower fT3 (about 0.4 lower) and so a lower fT3:fT4 ratio. They are designed to work this way, so their ratio should be lower. Since these people obtain a little more of their T3 from the blood they have more need for the T3 that is normally secreted from the thyroid.

In general I would ignore people who advocate fT3:fT4 ratios and not even bother to calculate them. These ratios are useful in rare circumstances in specialist studies.

It is useful to look at T3 levels. If a patient is given levothyroxine and consistently has a low normal fT3 for various fT4 levels within its reference interval then you can conclude there is impaired conversion. This often happens in patients with low normal TSH, fT3, fT4 (all three) and these patients do quite badly. I would say ignore ratios but do look at how TSH and fT3 respond to various fT4 levels. There should be a mechanism that keeps fT3 average and if this isn't happening the mechanism is broken.

Don't trust doctors who advocate ratios, they probably don't understand the underlying processes.

[Kriticat]

Wow, thanks everyone, that's so interesting and informative, constantly blown away by the knowledge on this forum... who needs an endo anyway?!