You can visit to your doctor ASAP. Min 4 Visits  needed for pregnant women It's always obstetrician call as sometimes behere are high risk pregnancies. Regarding you thyroxine 25 MCG,let the physician decide about modification

Firstly, the thyroid medication should be continued, moreover in late stages of pregnancy the dose needs to be increased due to increased demand. Secondly, ideally monthly visit is required during 1st 7 months, so you should visit a clinician within 1st month or if any symptoms develop. After 6 wks you should also do a USG along with other necessary investigations and TT injections, Iron, Folic acid, calcium tablets etc are to be taken. Also you should very importantly address the issues regarding your weight and age, as they are associated with some risks also. Also a fetal anomaly scan will be a good option later on.

You should continue your thyroid medications. You must visit the obstetrician at the earliest available time. You will be examined and will undergo blood investigations such as CBC, thyroid profile, RFT, LFT, serology tests, etc and you'll have your 1st dating scan. You will also be started with supplements