Low CBC

I am a 56 y.o. female with hypothyroidism.  I had some bloodwork done and my WBC is 3.5, my RBC is 3.89 and my platelets are in the normal range at 213, but from 265 last time.  Everything else is in range.  What could be the reason for the blood counts to drop, and should I be tested for anything maybe that might be causing this?  Also my thyroid panel is in range except for my Ft3 is slightly low.  I feel fine, have pretty good energy, no pains anywhere, sometimes light headed. Should I be concerned about this?

Thank-you