Suin, T3 should help raise FT3. FT4 may drop a little as T3 will also stimulate better conversion to T3, and TSH will drop lower too.
B12 is low but 1,000mcg should raise it. You should take a B Complex vitamin to keep folate good and the other B vits balanced.
VitD 97 is good but you will need 2,500iu D3 daily to maintain levels Oct-Apr when ultraviolet light is too low to stimulate vitD.
Ferritin is low. Halfway through range is optimal. Supplement Ferrous Fumarate and take each tablet with 500mg-1,000mg vitC to aid absorption and minimise constipation.
Suin, efgr >60 pretty much rules out kidney problems. The low CO2 may not be significant as your other U&Es are normal. The link suggests some possible causes, it's not a diagnosis. Discuss it with your GP if you are concerned and ask for a cortisol test.
Suin, NHS does serum cortisol. If it's below range it means adrenal insufficiency which can be Addisons. Private saliva tests show cortisol levels at 4 points through the day. I don't think NHS recognises saliva testing.
Suin, because they were normal 2 years ago doesn't mean they are today. Adrenals fail over time and are 90% shot by the time a crisis presents. If you don't want to show your GP the link how do you justify why you are asking for serum cortisol? Ask your GP what low CO2 means and whether it needs monitoring.
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