The thyroid sets the pace of metabolism, and the menstrual cycle is sensitive to it.
An underactive thyroid tends to make periods heavier and more frequent; an overactive one tends to make them lighter or absent. Both can affect ovulation, which is why thyroid function is usually checked when conception is difficult.
TSH is the usual first test, and it measures the signal the pituitary sends rather than thyroid hormone itself. A raised TSH generally means the thyroid is underperforming.
TSH alone does not tell the whole story. Free T4 and thyroid antibodies add to it, and what counts as an acceptable range shifts in pregnancy.