Thank you for your question. I can understand your concern. If your pregnancy test is negative you are unlikely to be pregnant. Thyroid disorders can upset your hormonal balance so that your ovary does not release an egg every cycle. This is the reason you do not get periods regularly. As your thyroid disorder gets corrected your periods will resume provided there are no other issues causing problems.  Not having regular bleeds at your age is of greater concern because it impacts your potential to conceive as well as risks of uterine ovarian and breast pathology. I would suggest you see a gynaecologist with expertise in endocrinology. In India these tend to be doctors specialising in fertility issues. They can help you design a plan to care for your thyroid problem as well as planning for pregnancy when you wish. Best wishes