Difference between T3 and T4 please 😊

[Tracyd32]

Hi, I’ve been feeling exhausted and run down but abe has other health issues. However, I’ve been on 100mcg of Levothyroxine for about three years. Saw GP today and discussed extreme tiredness etc. He is doing lots of bloods. I asked what the difference between T3 and T4 was and he said it doesn’t matter as they only test for T4. Can you help me understand what this means please? Thank you

[Hidden]

T3 is the 'active' form of thyroid hormone that your body uses, whereas T4 is a pro-hormone from which your body will make T3 as it needs. That's the ideal situation but for many people, this conversion from T4 to T3 doesn't happen efficiently and they can be left with inadequate T3. T3 has a very short half-life of around 7-8 hours compared to the much longer half life of T4. This means its effects quickly dissipate, and your body needs to keep making more.

The medical profession have decided giving T4 is easier and more effective (according to them!) since T4 has this longer half life and theoretically your body will simply convert it as required. If you don't have enough the dosage can be increased and that should put your lab results 'in range' which, by the medic's assessment, means you are pretty much ok and any symptoms at that point can't be due to your thyroid.

The sheer number of people who aren't well on T4 only should wave red flags from the rooftops but doctors mainly ignore that. It is very important to get some idea of where your T3 is at, and often labs won't test for it, particularly if they deem your TSH and FT4 are 'in range' and therefore 'normal'.

If you still have symptoms it's highly likely your free T3 is low in range and your body isn't converting enough, either because you need more T4 or you may have vitamin/mineral deficiencies that mean your body isn't utilising that T4 adequately. There are other possible reasons, too. Really you need, in particular, folate, ferritin, vit B12 and vit D3 testing, at the very least.

[Aurealis]

It means he’s fobbing you off or he knows nothing.... as Jadzhia explains.

[Eliotf]

GET A NEW DOCTOR ! And get your tests done for: Free T3, Free T4, T3, T4 and TSH. There are a few others that have to do with globulins. Jadzhia said it very well. Some people don't convert T4 to T3