Thyroid condition????????

i went to a diet dr the other day and got a prescription for phentermine and then had my blood tooken to check my thyroid he said if anything was wrong with it i would be sent a presctiption in the mail. So he sent me ..05 which is the lowest dosage they give for a slow thyroid. The prescription is Levothyroxine. im just afriad this will make me gain weight. Because thats the last thing i wanna do. any help from anyone would help