Here it comes, Biochemistry 101!
TSH (thyroid stimulating hormone) is the message that goes from your pituitary gland to your thyroid to stimulate your thyroid to make T4. TSH is not a thyroid hormone at all - it's a pituitary hormone. The higher the result of your TSH test, the more your pituitary is having to work to push the thyroid to do its job. Where I live the range is 0.35 -6, but they vary throughout the country, although not by much.
When stimulated by your pituitary your thyroid makes T4. This is what we replace with thyroxine, which is just synthetic T4. T4 is inactive. It is called T4 because it has, among other things, 4 iodine atoms attached to it. Your doctor will, if you're lucky, measure the free T4 which is going round in your blood. This shows in your results as FT4 and should be in the top third of the range, which for me is 9-26. OP is only a couple of points off the bottom of our range, which is not a good place to be.
The T4 loses one of its iodine atoms and converts to T3, which is the active hormone. Every cell in your body needs it and it is usually lack of T3 in the cells that gives you the symptoms. If you're lucky, your doc will measure your free T3 (FT3), which again needs to be towards the top of the range, which for me is 2.8-7.
If you are low in T4, which OP is, you will inevitably be low in T3 and usually feeling like .
At what level you will get treated depends on the range in your area, the level of knowledge of your doctor, whether you have an auto immune thyroid condition (and a knowledgable doctor) and a whole lot of other variables. I was told quite categorically by one doctor that I did not have a thryoid problem, and on my next visit was put on meds by another doctor. It can be a lottery.
Phew!
D