People with an underactive thyroid gland (hypothyroidism) often experience joint and muscle problems because of low levels of thyroid hormone. These problems may include muscle aches, tenderness and stiffness-especially in the shoulders and hips-joint pain and stiffness, and swelling of the knee joint and small joints in the hands and feet. Because symptoms can resemble those caused by early rheumatoid arthritis (RA), joint problems caused by hypothyroidism are difficult to distinguish from those resulting from RA. Both can cause painful, swollen, stiff joints, which are often worse after rest. But individuals with hypothyroidism generally experience less morning stiffness.