As per the subject title, if ft4 is inactive, then why do some people on T4/T3 therapy only feel good if their ft4 level is at the mid/upper level despite the fact their ft3 (active form) may be at a optimal level? Or if they are not feeling well, why are they advised to increase their Levo dosage if they have low levels and their ft3 is already at a optimal level on T4/T3 therapy.
Thank you for reading.
Hello, I hope we can help you.
Have you been diagnosed with a thyroid condition? It would help members to help you if you add some thyroid health information in your bio section
Hope this answers your question.....
...because we are all different!
Most people need a ( personal) optimal amount of T4 which can then be converted to T3 in the cells
Both Frees will then be roughly approaching 75% for good health
Some people have impaired T4 to T3 conversion and need replacement T3
FT3 in the serum, while still inactive, has to be transported into the cells where it reaches the nuclei, attaches to T3 receptors and becomes active in the cells/ tissues
Some people need a low dose of T4 along with optimal T3 to feel well.
Some people cannot tolerate levothyroxine and need T3-only to feel well
How we feel is an important part of diagnosis/ dosing.
There are no rules....dosing has to be done according to the individual's needs. It would be easier were it otherwise!
Hi there, t4 is inactive, but is converted into the active hormone t3. Some people don't convert t4 to t3 very well, which is why they add in some t3 in combination with their levothyroxine. However, its still a good idea to have t4, and not rely solely on t3 (although some people can't tolerate t4 at all, and do rely solely on t3) for a couple of reasons. Firstly, t4 has a longer half life, so its good to have a store floating about 'ready to go' when needed. T3 on the other hand only lasts 24 hours so has to be taken more frequently. The other reason for having some t4 as well as t3 is that some organs prefer to 'make their own' t3 from t4, whereas other organs prefer their t3 delivered, ready made. As to your question about why increase t4, well its really about trying to find the optimal levels for the individual for both t4 and t3. If t3 is already quite high, the safest option is often a slight t4 increase.
Thank you
Just curious, any idea which organs like T3 delivered, and which like converting the T4 to T3? I have a barely detectable TSH on 100 levo. and 15 g of T3 which tend to produce bloods with FT3/4 in 40% range, and no one willing to prescribe more hormone replacement because of dismal TSH. Looking back at my old test results I never got both FTs together up any further…they are like those traditional two weather forecast figures when one comes out, the other goes in, ie FT3 higher in 60% FT4 in 20%, and vice versa!
The heart and liver like to have t3 delivered direct, the brain prefers to convert t4 to t3.... possibly due to having to pass through the blood/brain barrier?
. There's not too much research out there, still early days, but a few comments I've noticed in a few recent papers.
And how do you feel? Which bits aren't working right?!
Unfortunately I have many other health ailments made worse by a number of Long Covid ones relating to high BP/HR and faints,but even in this context last week got the falls consultant having a go at my low TSH! My brain could do with a boost, perhaps I’ll think about a bit more levo.eventually!
It's not 100% certain that T4 is inactive. The jury is still out on that one. It doesn't do much, that's sure, but it could be that it does more for some people than for others.
My usual FT4 is in the bottom quartile and I'm on NDT. I've never been able to get it above 50% of the clinical range. Suppressed TSH alarms the endocrinologist. He panics. He sweats. He's afraid my heart will explode. He wants to reduce my dose.
When I experimented and suspended treatment for two days to evoke a TSH that would make the endocrinologist happy, I felt awful by the end of the second day.
Now imagine a catastrophe where I have to evacuate and can't get more medication. I keep a healthy supply of NDT in my Go Bag.
There would be a lot of us going downhill fast if there was a major catastrophe. I need my migraine medication too. And without my HRT patches I’d be ready to attack anyone that even looked at me in the wrong way 😂
My dose is 75 mcg T4 and 25 mcg Liothyronine. My FT3 is 3.9 upper part of range and my FT4 is .8 which is bottom of range. It has been like that for about 5 years now. I feel good and T3 helped me immensely. I was in a world of hurt before T3 was added. I believe it is what suits the individual best. We are all very different and should be treated as such.
Why do we still need T4 when taking T3
T4 Monotherapy Versus T4/T3 Combination Therapy
Studies on TSH for those on combined T3 T4 therapy 
Examining the need for T4/T3 combination therapy
T4 & T3 combination therapy... day 9... awesome! 
