It depends on if you have an underactive or overactive thyroid. Overactive means your glands makes and releases more hormones than you need. Underactive is the opposite. Overactive can result in weight loss, irregular heart beat, irritability. Underactive can result in weight gain, feeling tired and weak, and can lead to heart attacks or stoke if bad.
I am on medicines proscribed by my doctor for my underactive thyroid. You have to get blood work done every 6 months to monitor your levels. Hope this helps!