It depends whether lowering Levo will raise your TSH. It doesn't seem to happen with me but I've been on Levo for 40+ years (last 2 years T3 added) and the last time my TSH was in range was 2003. My FT4 plummets if I lower my Levo to try and raise TSH and I feel extremely unwell. But that might not be the case with you.
TSH can lower when adding T3. But at the end of the day what matters is how you feel, where you feel best and the dose that keeps you there. Low TSH isn't a problem according to Dr Toft, leading endocrinologist and past president of the British Thyroid Association, as long as FT3 stays within range. See thyroiduk.org.uk/tuk/about_... > Treatment Options:
According to the BMA's booklet, "Understanding Thyroid Disorders", many people do not feel well unless their levels are at the bottom of the TSH range or below and at the top of the FT4 range or a little above.
The booklet is published by the British Medical Association for patients. Avalable on Amazon and from pharmacies for £4.95 and might be worth buying to highlight the appropriate part and show your doctor.
Dr Toft states in Pulse Magazine, "The appropriate dose of levothyroxine is that which restores euthyroidism and serum TSH to the lower part of the reference range - 0.2-0.5mU/l.
In this case, free thyroxine is likely to be in the upper part of its reference range or even slightly elevated – 18-22pmol/l. Most patients will feel well in that circumstance.
But some need a higher dose of levothyroxine to suppress serum TSH and then the serum-free T4 concentration will be elevated at around 24-28pmol/l.
This 'exogenous subclinical hyperthyroidism' is not dangerous as long as serum T3 is unequivocally normal – that is, serum total around T3 1.7nmol/l (reference range 1.0-2.2nmol/l)."
You can obtain a copy of the article by emailing firstname.lastname@example.org print it and highlight question 6 to show your doctor.