Since being on lithium my tsh has gone from 1.5 to 8.4. what does that mean? i already take 25 mcg of levothyroxine.

Lithium: Your elevation of TSH means that your thyroid gland is under-functioning, or hypothyroid. This is a known side effect of lithium. Please talk with your endocrinologist and psychiatrist about this -- it is severe enough that at the very least, you'll need to adjust your thyroid medication if not possibly change to a different mood stabilizer.