When the immune system attacks the thyroid, the gland is slowly destroyed causing it to spill excess thyroid hormone into the bloodstream. This excess thyroid hormone overstimulates metabolism, causing symptoms of hyperthyroidism. (Anxiety, insomnia, a pounding heart, or nervousness, and more.)
Over time as the gland is continually destroyed, it loses its ability to secrete sufficient thyroid hormone, causing symptoms of hypothyroidism. (Fatigue, weight gain, headaches, depression, constipation, poor circulation, digestive problems, hair loss, and more.)
Most cases of Hashimoto’s involve a slow but steady destruction of the gland with gradual worsening of hypothyroid symptoms.