An underactive thyroid can cause weight gain, depression, muscle aches, fatigue, dry skin and dry or brittle hair, hair loss, constipation, a constant feeling of being cold, and many other symptoms.
Hope this helps.
Your thyroid affects how your metabolism works. If your hyper you often lose weight and if your hypo you often gain since your metabolism is slowed down. Being onthe right dose of meds helps with this problem but it's hard to get the right dose sice each person is different. I know some who feel fine with their TSH at 5 while when mine was at 4.8 I was having a hard time functioning. LL