The thyroid gland regulates calcium homeostasis through a complex interplay of parathyroid hormone (PTH), calcitonin, and calcitriol (vitamin D). PTH increases blood calcium by promoting bone resorption and renal reabsorption. Calcitonin decreases blood calcium by inhibiting bone resorption. Calcitriol stimulates intestinal calcium absorption. Together, these hormones tightly control blood calcium levels through effects on bone, kidney, and intestine. Imbalances can lead to hyperparathyroidism, hypoparathyroidism, or vitamin D deficiency disorders.