Treatment for an underactive thyroid involves taking daily hormone replacement tablets, called levothyroxine, to raise your thyroxine levels. You'll usually need treatment for the rest of your life. However, with proper treatment, you should be able to lead a normal, healthy life. If an underactive thyroid isn't treated, it can lead to complications, including heart disease, goitre, pregnancy problems and a life-threatening condition called myxoedema coma (although this is very rare).