Hi,
I too have suffered from these symptoms. I am on Thyroxine for the thyroid, its a small tablet each day, but also on Metformin. I started them at different times and it was the metformin rather than the thyroxine that gave me my energy back, drop in appetite and weight loss (small and steady loss).
However, I have been taking iron for the anemia - I take Spatone - its in liquid form and you can get it at Tesco, Waitrose and Boots. Its a natural form of iron, incredibly effective and most importantly no impact on your bowel movements .....
I didn't take B12, however your metoblism of b12 is effected by the thyroid and this in turn effects the iron absorbtion, in fixing the thyroid my B12 levels have improved without intervention.
Everyone is different, but the important thing is the thyroid commonly causes undiagnosed problems in women, you should see a specialist rather than your GP if this is suspected, and it can effect your diet, metoblism, energy, absobtion of vitamins, all sorts.
With a dodgy thyroid, supplements for deficiencies such as iron are often your only choice as you'll not get enough from food sources. (told that by several doctors). Thryoid conditions are linked to fertility and miscarriage and can sometime mask other endocrine disorders so worth seeing a specialist to get it check out correctly.