Hi again Sal333,
after rereading your original post & looking at your TFT results I'm sorry but the picture is a bit muddy

! So I have some more questions if that's OK.
Low folate can be a consequence of hypothyroidism & I think both conditions could give rise to the symptoms you describe here
fatigue, tingling/numb fingers/toes, hair loss,aching muscles, low mood, brain fog etc but what finally made me visit Gp was the racing heart/shortness of breath which would happen just sitting down doing nothing
But the low folate finding is making it difficult to get an idea of whether it's straightforward hypothyroidism behind the lot (the folate included).
So you were prescribed a 3-month course of folic acid from Feb, then that was stopped, is that right? Was your folate tested & found to be back within range at the end of the course? I'm wondering whether your GP is wanting to see whether treating the thyroid will keep your folate within range for the future, which hopefully it should given adequate thyroid replacement over time, assuming there's no other reason for the low folate.
Your Free T4 has been lowish for around 4 years then, according to the results you posted & your TSH has wavered in & out of range over the same period. That's a long time to have a fairly low level of thyroid hormone in your system so I'm not surprised you've felt pretty under par

. But it's standard practice not to treat while TSH is still more or less within range as no-one wants to start a patient on lifelong medication if there's a chance the condition is temporary. I think your GP was right to start you on Thyroxine recently though, given your persistent symptoms & TSH above ref range over several months.
Can I ask do you have other signs of low thyroid such as weight gain, fluid retention, disproportionate coldness, eyebrow loss or sluggish tummy?
Your latest TSH @ 3.15 might not be low enough for you to feel well. Some patients, especially when younger, do best with a pretty low TSH, but it's very individual & a good doctor will take into consideration how you feel rather than just going on blood results. Increasing your Thyroxine slightly might improve your symptoms, but you would need to do this with your GP's approval. A slight change in dose can have quite a marked effect on blood results & on how we feel. As you are already on a very low dose it could be tricky increasing it without giving you too much, but I wonder whether another 12.5mcg every 3rd day might bring your TSH down over time, enough for you to feel better. If not, then with your GP's agreement maybe 25/37.5mcg on alternate days. If your 25mcg tablets are scored you should be able to split them with a tablet splitter (from a pharmacy) to get the 12.5mcg dose. Alternatively if you are on TEVA brand these come in 12.5mcg tablets so no faffing around or wastage trying to split crumbly T4 tablets into halves!

Another thought first - did your GP ask you to have the bloods done before you took that day's dose of Thyroxine? My Endocrinologist's standing instructions are no thyroid medication on day of testing until afterwards. This is thought to give a better indication of background thyroid levels. In other words if you had your thyroid meds that day before the test, that would likely have given a lower TSH result & had they tested it, higher T4, not doing you any favours if you need a higher dose of replacement to be asymptomatic.
I think the reason they didn't test T4 last time was probably because your TSH was back in range & they would assume this is because the Thyroxine is correcting your low T4 level. TSH alone is often considered enough if that's within ref range, but I agree it would be reassuring to know the T4 level especially in the initial stages of treatment.
It is unfortunate the NHS rarely tests T3 for hypo patients these days, as there is a small subgroup (around 15%) who don't convert Thyroxine into the active form very efficiently & that can leave us with persistent symptoms though our TSH & T4 show optimal levels. If you find that getting your TSH down further doesn't make you feel better in time & your folate has been corrected, I would push for a T3 test & if your GP's hands are tied on that I would try to go private for one if that's within your means.
Wx