The following graphic represents the control of calcium ion levels in the blood: The figure portrays the regulation of calcium ions in the blood. Low blood concentrations of calcium ions stimulate the parathyroid gland to increase secretion of PTH, which acts on the kidney, large intestine, and bone to increase blood calcium levels. Once high levels of blood calcium are reached, the thyroid gland is stimulated to secrete calcitonin, which stimulates the bone and kidneys to decrease blood calcium levels. Concurrently, the high blood levels of calcium inhibit the parathyroid, which decreases PTH secretion, which lowers blood calcium levels. Which statement is true if PTH is removed from the system upon secretion, resulting in negative feedback