How to tell if converting well from t4 to t3

[Meadowsmom]

What does one look for in blood lab test results to tell if proper conversion is taking place from t4 to t3? I am getting blood labs taken next week (on t4 only med now) and need to know what to look for to determine if I am converting well. Thank you!

[SeasideSusie Remembering]

Look at the FT4 and FT3 results and reference ranges.

If FT4 is at top of range and FT3 at bottom of range then conversion is poor.

It's possible to work out percentages through range as well, so post your results/ranges when you have them and we can comment.

[Meadowsmom in reply to SeasideSusie]

Ok. Where should both t3 and t4 ideally be, top of the range? I will post results when I get them. Thank you!

[SeasideSusie Remembering in reply to Meadowsmom]

When on Levo only, the aim of a treated hypo patient generally is for TSH to be 1 or lower with FT4 and FT3 in the upper part of their ranges if that is where you feel well.

[Heloise]

Levothyroxine is only T4. After conversion T4 turns into either T3 or reverse T3. There is debate whether measuring reverse T3 is helpful or not but it would be good to know what your FT3 level is.

[RockyPath]

Reverse T3 and ita ratio to FT3 is an excellent indicator, though sometimes the rT3 may simply be above the reference range.

Usually you can simply tell by how awful you feel. There is a circadian rhythm to the conversion process and you may be better for a few hours twice per day and experience overt hypothyroid symptoms the rest of the day.

[milkwoman in reply to RockyPath]

Can you explain what the ratio ideally should be? I’m also currently trying to determine if T3 would help me.

[RockyPath in reply to milkwoman]

When you receive the results, you look at the ratio of FT3 to rT3, doing a simple division, (FT3 divided by rT3), ignoring the different units of measure. A result that is a small number is an indication of too much rT3. You can have FT3 within laboratory reference range that is normal but if the ratio of FT3 to rT3 is .11 (for example) you would be experiencing various hypothyroid symptoms, including fatigue, slow heart rate, brain fog. At .2 you would start to feel better. The units of measure at your lab might be different, so the results might be 1.1 versus 2.0, or 11 versus 20. There are articles out there discussing the ratio and the magic of why you don't need to have identical units of measure.

The SHBG test shows how much T3 is reaching your cells. It will be in the low end when cells are hungry andrise when you’re getting enough.

[milkwoman in reply to RockyPath]

Thank you for the explanation. It does look like I am a poor converter and could use T3.

Isn’t SHBG “Sex hormone binding globulin?”

What does that have to do with T3?

[RockyPath in reply to milkwoman]

It's known among clinicians who understand the endocrine system as an indicator of cellular T3 action.

[RockyPath in reply to RockyPath]

My endocrinologist didn't volunteer to test for it, but when I mentioned that it would indicate T3 availability to the cells, he agreed. Now he tests me for it each time.

[RockyPath in reply to RockyPath]

If you're not converting well, your doctor should be reducing the LT4 and making room for more LT3.

[milkwoman in reply to RockyPath]

Very interesting about SHBG. I will ask my endo about it! Thx!

Found this article:

google.com/amp/s/www.holtor...

[RockyPath in reply to milkwoman]

That's a good one!

[milkwoman in reply to RockyPath]

I agree!!

[Dadondadda]

I had this issue myself. It wasn't easy. You want to see how much T3 is getting to cells and there is no test for that.

I read all the textbooks and studies on this for years. Serum T3 is prioritised over cellular T3 (so cellular can be much worse) and there can be conversion derangement which does not raise rt3, oddly.

If I have doubts now I test the following:

Total t4

Total t3

Free t3

Reverse T3

I found ft3 to be sensitive when low but insensitive when high.

I found ft4 meaningless and in a few studies it was inversely correlated to cellular T3 and also to insulin sensitivity.

It also seems tsh is most meaningful on t4 only. Malign it all you want but most of the studies demonstrating the useful of thyroid signalling on critical biological processes simply used tsh.

I don't think there is much meaning in percentiles in ranges personally.

Ultimately the clearest signals for me were:

Total t3 less than 100

Rt3 high outside of range - but this could also mean overmedication.

Low ft3

High tsh despite high dose but this fluctuates during the day.

You can also test some of the major determinants of conversion directly but this can be expensive and time consuming.