Fnp wants to raise my synthroid (thyroxine) from 50 mcg to 75. i don't understand the mechanics behind this medicine. can you explain?

TSH: Hi. Your FNP probably tested your TSH and found it to be high on your current dose of 50 mcg/day. TSH is the pituitary hormone that reflects thyroid status (assuming normal pituitary function); if TSH is hi, your pituitary is trying to push your thyroid to make more thyroid hormone, but your thyroid is damaged and can't. Therefore, you give more Synthroid (thyroxine) until the TSH is down in the normal range