Calcitonin

Amongst the thyroid epithelial cells are other specialised cells that produce calcitonin. Calcitonin is a hormone produced by the parafollicular cells.

Calcitonin acts to reduce blood calcium levels and works opposite to parathyroid hormone (PTH) from the parathyroid glands.

Calcitonin reduces calcium absorption by the gut, inhibits the break down of calcium within the bones, increases urinary excretion of calcium and regulates vitamin D.

However, it is the view of endocrinologists that neither high, nor low calcitonin, presents any real issue. It is thought to have only a minor effect. 

So, it ought not to be a concern to those of us who have lost all thyroid tissue due to Hashimoto's thyroditis or through total thyroidectomy.