This can happen following a thyroid gland removal surgery when the parathyroids are accidentally taken out too or radiation treatment for cancer of the head or neck.
If calcium levels fall, the four parathyroid glands buried within the thyroid gland release parathyroid hormone which frees up both calcium and phosphate ions from the bones.
One example is thyroid hyperplasia, an enlargement of the thyroid gland caused by an abnormal rapid growth of the epithelial cells lining the follicles.
Now, we don't fully understand why yet, but if you have something like Graves' disease, your body produces abnormal antibodies that target the cells of your thyroid gland itself.