Need second openion. We had confusion!

She wants to conceive pragnency, We are trying for last 4 months and having irregular periods, last month she got 30 days cycle, a regualar one, we recently consulted a gynecologist she asked for a thyroid test, we had done that as well. TSH level in report is 5.38 , doctor advised to take Thyoxine sodium tablets. My concern is that whether she needs to start medicine of thyroid or not, bcoz I came to hear that thyroid sodium tablets are harmful for infant. Also we had confusion regarding thyroid report as well, bcoz some reading suggests that thyroid should be between 0.40-5.50 and some recommends 0.40-4.50..Also my concern is once she start taking medicine will not be it become habitual and neccessary for her to control thyroid. Kindly suggest.