Untreated hypothyroidism can lead to a number of health problems such as:myxedema, coma, goiter(a swelling of the gland due to constant stimulation of the thyroid to release more hormones causing the gland to become large), heart problems, mental health issues, infertility because low levels of thyroid hormone interfere with ovulation and birth defects to babies born to women with untreated thyroid disease.
Attribution: Straight Talk Foundation