One of the most common reasons for a missed period is pregnancy. Therefore, you should perform a pregnancy test to determine if your missed period is due to pregnancy.
With regards to thyroid medication, discontinuing thyroid medication (whether it be for hyperthyroidism or hypothyroidism) can affect your periods and lead to missed periods – mainly occurring with hyperthyroid patients. Hyperthyroidism is an overactive thyroid. Although most cases are asymptomatic, excessive thyroid hormone can affect your menstrual cycle by decreasing the frequency of your periods and making your periods lighter. In severe cases, hyperthyroidism can cause menstruation to cease.