Thyroid drug has long half like 2 weeks. Since you were asked to repeat it to see your body's response you can give blood after taking it or with out it. But you have to give your blood in fasting before 8am. It is always better to do the profile after screening it with TSH. Because free T4 level will be a better parameter to monitor since TSH will little bit more time to come down. Do always TSH third generation assay which is more sensitive and specific.

Hi....Mr.Lean...,
welcome to HCM...,
Thanks for query...,
Regarding thyroid tests.......,
1) T3 active only few hours in the days...,
2) T4 has long period of activity....,
3) TSH tests reflects the impact of the several weeks of medication...,
here TSH levels (after eaten) likely to have lower TSH levels than if test ...
earlier in the day with fasting....,
hence do not take medication before taking blood....,o.k good luck...,

Hello.Welcome to HCM forum.Whether you take the tablet or not ,it doesn't make any difference.Thyroid produces mainly T4 which is 5 fold less active than T3. 85% of T3 is formed from peripheralconversion of T4.Most T3 and T4 in blood is protein bound.The unbound or free portion is the active part.Ask for only T4 and TSH .Thanks and good luck.

The half life of eltroxin is 7 days. Hence For monitoring there is no need to stop the drug. We advice people to give blood between 7 am - 8 am in the morning in fasting and eltroxin won' t produce any changes with in an hour or two. If the patient is on eltroxin for quite long time then it is ideal to stop it for a week before testing. Always it is ideal to give the list of drugs taken by the patient for proper interpretation of results to the lab eg. Ameodarone. Iam an biochemist(M.D bio) and we used to standardize before introducing the test.