Dosage is based loosely on body weight and degree of thyroid failure. TSH really has little to do with it. Someone with a starting TSH of 100 and someone starting at 5 each usually ends up on roughly the same dose.
The average full-replacement dose for an adult is 120 mcgs of synthetic med or 120 mgs of natural dessicated.
TSH has very little to do with thyroid function. It's a pituitary hormone. The med replaces the actual thyroid hormone.