For anyone on here who hasn't already been on there, the thyroid UK website is really good. It explains all about TSH, T4 and T3 tests, and also the tests to determine whether it's autoimmune based.
It also goes into detail about the different kinds of thyroid disease, how they effect you etc.
There are also lists of providers if you are interested in using private blood testing companies. I've done this regularly for various reasons (and it was a private test that actually told me I had a problem with my thyroid in the first place, then a subsequent one that showed I had the antibodies confirming I have Hashimoto's disease).
It talks about medication, and there is a section about liothyronine (T3 medication), which is what I'm on (levothyroxine didn't work for me, I just felt like a zombie). Probably best just to focus on levothyroxine for now, you don't want to over complicate matters and hopefully you'll do just fine on it. If you don't (and it can take a long time to get to the right dose) then just know that there are other options out there.
There is also a link on there to the thyroid section of the Health Unlocked forum. It is full of some very knowledgeable people, who pretty much saved my life I think in relation to my thyroid disease. You need to understand that the majority of GPs have very limited knowledge of thyroid disease - for example, they will test TSH alone and tell you that you are 'over medicated' without testing your T4 and T3. The T3 test is vital, because T3 is the active hormone, so they really can't get a full picture of your thyroid without it. Also, most endocrinologists are diabetes experts rather than thyroid experts, and some can be as poor as the GPs are. I've experienced that myself, and sadly the Health Unlocked forum is full of people who've had the same.
Just picking up a few things the op has said - heavy bleeding can absolutely be caused by underactive thyroid. I had to have an ablation because of my bleeding, and if I remember rightly it was in between seeing gynaecology and having the op that I got my thyroid results.
Being autoimmune, there is evidence out there that shows going strictly gluten free can help. Ideally you should have a coeliac blood test first (because you need to be eating gluten for 6 weeks for at least 2 males a day) to rule that out, as it's very common to have more than one autoimmune disease and coeliac disease and Hashimoto's go hand in hand.
Really important as other people have mentioned to make sure your vitamins and iron are in a good place. And that doesn't mean just in the range, it means optimally in range, so generally in the top quarter of the range. Specifically you want to focus on vitamin B12, vitamin D, folate and ferritin. I had very low ferritin early on in my journey, and was losing hair. Underactive thyroid can also be a cause of this, so it could have been either, but I managed to raise my ferritin and the hair loss stopped.
I put on a lot of weight, and unfortunately, I guess because thyroxine didn't work for me, I didn't start to be able to lose it until I added T3 medication into the mix.
I cannot emphasis enough to go on the Health Unlocked forum to anyone on here struggling with their thyroid. I've learnt so much on there, including how to advocate for myself. It gave me hope when I felt like death on thyroxine, and the tools I needed to learn about other medications. I actually get my T3 prescribed on the NHS, which took a huge fight and a number of years, but was worth persevering!
Good luck with your endo appointment on Wednesday. If it goes poorly, assume they're a diabetes expert rather than thyroid! If that's the case, you can ask for recommendations over on Health Unlocked and ask to be referred elsewhere. My first endocrinologist was awful, I now go to one in a different county. Our first meeting wasn't brilliant, in fact I complained to PALS about him! However, we came to an understanding, he acknowledges that I've researched things properly, and have done all my medication changing and raising doses slowly and steadily. Whilst we tend to argue a bit during appointments, the letters he writes to the GP are actually very supportive, so I'll let him off!