Parathyroid hormone secretion from the Parathyroid glands increases extracellular Ca⁺ levels by causing bone ⁺ reabsorption and increased Ca⁺ uptake in the kidney. Parathyroid hormone increases vitamin D synthesis. Calcitonin, secreted by the thyroid glands, inhibits bone reabsorption and lowers blood Ca⁺ levels when they are too high.