10 days should in theory be long enough for the added T3 to have left your system. But that does not mean it's long enough for your TSH / fT4 /fT3 results to revert to their 'natural' state .
TSH can often take a few weeks (at least) to settle back to it's normal .. and as TSH changes , it affects the thyroid's production of T4 and T3 .. and it also affects the conversion of T4 to T3 in the body. So definitely the TSH and possibly the fT4 /fT3 results could all be expected to continue changing for a lot longer than 10 days .
As long as this is borne in mind there is no reason you can't test @10 days .. but if those result don't get you what you want ,, then insist on retesting after 6-8 weeks.
Not quite .. T4 has a half life of about 7 days in blood .
T3 has a half life of about 1 or 2 days in blood. (can't remember exact time off top of my head) this means after 1 or 2 days 'half of what you took has gone' (less any you used) . after another 1 or 2 days 'half of that remaining half has gone' (less any that you used)
But once within the cells . T3 continues to have it's action for 2 or 3 days . Hence some poeple are fine taking it in one daily dose . The reason for split dosing of T3 is to smooth out the effect for those that feel it , not everyone does and to moderate the highish fT3 levels in blood that happen when a full daily dose is absorbed from the gut all in one go.
I follow a scientist/researcher/doctor who took T3 himself and he advised taking one daily dose of T3 with a glass of water that saturated all of the T3 receptor cells and stated that the effect of that one dose can last up to three days.
DrLowe is now a deceased scientist (due to an accident). His name was Dr John Lowe, who was also an Adviser to TUK (thyroiduk).
I trialled that myself by taking my usual one daily dose of T3 and toook none on day 2 or day 3 and, yes the one dose lasted three days.
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