Hypothyroidism can cause all kinds of aweful symptoms. Your thyroid is one of your master glands. It controls your metabolism....respiratory system....heart.....hormones.....etc.
So it can affect your entire body and will make you very ill if left untreated with low levels.
If you fall too low for too long of a period of time, it can cause a myxedema coma and can even kill you. Nothing to fool around with.
When low in your active form of T3 ( Free T3) the body is slow. Less able to preform at the optimal level.