T3 is available by the body for
use almost immediately. It is used up
in hours unlike T4 which takes days or weeks. So
in order to try to give you a bit better constant level it makes sense to take multiple times throughout the day. Be careful not to take the 2nd dose too late in the afternoon as it may affect your ability to get to sleep.
Please get reference ranges for your lab tests.
The T4 medication seems to be directly linked to some symptoms of headache.