The normal course of medication is to start with the lowest dose i.e. 50mcg/day and increase or adjust
every 3-4 weeks until normal metabolism
is steady, your TSH level
remained too high with 50mcg/day dose, so your Doc increased it to 75mcg,
by the way normal TSH value is 0.3-3(before 2003 it was
0.4-6) your doctor will adjust your dosing till all is steady and well.