It is very disappointing that an Endo displayed such a lack of knowledge about thyroid med. How long it takes to leave the system is not the issue regarding when to take thyroid med relative to the blood draw. The issue is how quickly it affects serum levels. Even the ATA/AACE Guidelines for Hypothyroidism provides a recommendation. Following is a link.
In the link, on page 999 it states:
"In monitoring patients with hypothyroidism on L-thyroxine replacement, blood for assessment of serum free T4 should be collected before dosing because the level will be transiently increased by up to 20% after L-thyroxine administration (72). In one small study of athyreotic patients, serum total T4 levels increased above baseline by 1 hour and peaked at 2.5 hours, while serum free T4 levels peaked at 3.5 hours and remained higher than baseline for 9 hours (72)"
So, to avoid false high results, which could affect the doctor's decision about your dosage, it is best to delay your thyroid med dose until after the blood draw for tests for FT4 and FT3.
My doctor told me not to.
I just went to another doctor to have antibodies tested. When the tests came back both high I showed them to the doctor who refused to test these labs. Sour face anyone? lol Last time I saw that Doc.
Oh I forgot..that sane doctor who refused to test my antibodies did test my vitamin D as I requested. Said no action needed. I requested my lab work to make sure as I trust no doctor. I had vitamin D deficiency. smh