What's the exact result of your FT4, along with its reference range? It's beginning to sound like you could have a conversion issue, but that depends on your actual FT4 level, since it can be within the "normal" range and still not be high enough for you.
Vitamin B-12 deficiency can cause a lot of symptoms that can resemble some of those associated with thyroid. As with most tests, the range can vary from lab to lab, yeah - many have a range of 200 - 900 or 1100. Again, B-12 needs to be at the top of its range, so even if your result is in the "normal" range, it's not enough for you.
Vitamin D of 16 is way too low. That needs to be 50-60.
You need to start supplementing, both B-12 and D to bring your levels up. Get a methyl B-12 as that's the form the body needs. Get a chewable, sublingual, or liquid, as they're more easily absorbed. Make sure you take it every day. You could start out with around 2000 IU/day of Vitamin D, then retest in a couple of months to see if your level is coming up.
You'll need to talk to your doctor about ordering Free T3 and Free T4 - every time you have a TSH test, since TSH doesn't necessarily correlate with actual hormone levels. Also ask your doctor about testing Ferritin. Ferritin is the iron storage hormone, so that will tell you whether or not you have adequate iron stores in your body.
What’s the reference range for the FT4? Ranges vary from lab to lab so have to come from your own reports. Have you had an FT3 (Free T3) test done? Free T3 is the hormone our individual cells actually use. FT4 is considered a storage hormone that must be converted to FT3 prior to use. Some of us have problems converting FT4 to FT3 and have to add a source of T3 to our daily regimen. This can be in the form of a T3, such as Cytomel (Liothyronine) or a T4/T3 combo, such as Armour thyroid.
Have you had Vitamin B-12 and Ferritin tested? Both of those, along with Vitamin D are necessary for proper metabolism of thyroid hormones. If you have “non-existent” levels of vitamin D, that could be one of your problems, since vitamin D deficiency can also cause other symptoms.
For now, we need to know: the Range for your FT4
Have you had an FT3 test
Have had vitamin B-12 and ferritin tests
If you’ve had any of these tests, please post results. Vitamin D needs to be at least 50, B-12 needs to be at the top of its range do avoid symptoms of deficiency and ferritin should be around 100.
I should also add in I’ve had almost nonexistent levels of vitamin D the entire 6 years as well!