Hi - well it really depends what issues you have currently and what tests you have had to date. Have you any reason to suspect that you might have diabetes - high blood sugar reading at any point, extra thirst, etc? If not then that would negate the need for Hba1c and Insulin, anyway GPs are usually quite keen to check these if there is any suspicion (I believe they are incentivised on this), so you could get them free there. Because of the way the body treats magnesium, favouring the blood supply to keep levels constant there, I believe it is very rare for a test to show any issues, even with significant deficiencies, so that is of no value. So that leaves us with reverse T3 - that always demands a venous blood draw btw, rather than finger-prick so you may well also be out another charge to have blood drawn.
So - RT3 - have you had earlier blood tests that show all levels optimal (including nutrients) yet you still feel hypo? In that case rt3 may be useful as, whilst all numbers can look good, a high rt3 could block the t3 receptors in cells. This stops your body using the ft3 in the blood to drive its systems so your cells would still be hypo despite having an apparently decent level of ft3. This is because rt3 is the body's brake, enabling it to slow down all systems to force a rest period, for instance during illness, infection or severe inflammation.
Active B12 can be tested now, though I haven't seen it bundled with thyroid tests, it has to be added on. This is useful if you have been supplementing with b12, blood numbers look good and yet you still have deficiency symptoms.
That is probably as clear as mud, but I hope it helps in some way.
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