Thyroid hormones are essential for the healthy function of many body systems. Therefore, when Hashimoto's disease and hypothyroidism are left untreated, many complications can occur. These include:

Goiter. A goiter is enlargement of the thyroid. As thyroid hormone production declines due to Hashimoto's disease, the thyroid receives signals from the pituitary gland to make more. This cycle may result in a goiter. It's generally not uncomfortable, but a large goiter can affect your appearance and may interfere with swallowing or breathing.

Heart problems. Hypothyroidism can result in poor heart function, an enlarged heart and irregular heartbeats. It can also result in high levels of low-density lipoprotein (LDL) cholesterol — the "bad" cholesterol — that is a risk factor for cardiovascular disease and heart failure.

Mental health issues. Depression or other mental health disorders may occur early in Hashimoto's disease and may become more severe over time.

Sexual and reproductive dysfunction. In women, hypothyroidism can result in a reduced sexual desire (libido), an inability to ovulate, and irregular and excessive menstrual bleeding. Men with hypothyroidism may have a reduced libido, erectile dysfunction and a lowered sperm count.

Poor pregnancy outcomes. Hypothyroidism during pregnancy may increase the risk of a miscarriage or preterm birth. Babies born to women with untreated hypothyroidism are at risk for decreased intellectual abilities, autism, speech delays and other developmental disorders.

Myxedema (miks-uh-DEE-muh). This rare, life-threatening condition can develop due to long-term, severe, untreated hypothyroidism. Its signs and symptoms include drowsiness followed by profound lethargy and unconsciousness. A myxedema coma may be triggered by exposure to cold, sedatives, infection or other stress on your body. Myxedema requires immediate emergency medical treatment.