Is it normal to feel dizzy and weak 4 days after hemi-thyroidectomy? when do i get concerned about hormone replacement?

Not uncommon: It is not uncommon to feel somewhat weak after any procedure that requires general anesthesia. If you had only a hemithyroidectomy, the remaining thyroid lobe should be adequate to make enough to meed the needs of your body for thyroid hormone, but it is important to have periodic blood tests to be sure that your levels are adequate.