Most people who take Cytomel split the dose
and take half with their Synthoid in the morning
and the other half sometime in the afternoon. Cytomel is very fast-acting, so if you don't split it, you can experience peaks and valleys in your levels throughout the day. You can experiment with when it's best for you to take the second half dose, but most people try to take it before about 3:00 in the afternoon (depending on schedule, of course) so that it doesn't disturb sleep.