Most people wouldn't feel well with a TSH of 4.21.
The aim of a treated hypo patient generally is for TSH to be 1 or below or wherever it is needed for FT4 and FT3 to be in the upper part of their respective reference ranges if that is where you feel well.
If you are still symptomatic then you should ask for an increase in dose.
You are under medicated. The goal of Levothyroxine is to restore the patient to euthyroid status. For most patients that will be when TSH is 0.35 - 1.0 with FT4 in the upper range. FT4 needs to be in the upper range in order that sufficient T3 is converted. Read Treatment Options in thyroiduk.org.uk/tuk/about_... Email louise.roberts@thyroiduk.org.uk if you would like a copy of the Pulse article to show your GP.
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