How long does it usually take untreated hypothyroidism to cause serious even fatal complications?

A long time: But it depends upon many factors, such as age, severity, other concurrent disorders, etc. It's easy to get it diagnosed and treated. See you doctor soon.

No problem if treat: Left untreated, hypothyroidism can lead to multiple complications and health problems. Markedly enlarged gland or goiter can occur. Increased cholesterol and heart disease develops. Long-term, untreated hypothyroidism, myxedema, occurs when hormone levels extremely low. Its symptoms include intense cold intolerance and drowsiness followed by fatigue/lethargy and unconsciousness (myxedema coma).