Question

regulation of calcium levels
increase in blood ca²⁺ stimulates ________ secretion, which leads to ________.
thyroid hormone; less urinary calcium excretion
parathyroid hormone; increased osteoclast activity
calcitriol; more urinary phosphate excretion
calcitonin; more urinary phosphate reabsorption
growth hormone; increased osteoblast activity

Explanation:

When blood calcium ($Ca^{2+}$) levels increase, the thyroid - derived hormone calcitonin is secreted. Calcitonin acts to lower blood calcium levels. One of its actions is to increase urinary phosphate reabsorption, which helps in calcium regulation. Thyroid hormone is not directly involved in this calcium - regulating response to high blood calcium. Parathyroid hormone is released when blood calcium is low. Calcitriol increases calcium and phosphate absorption. Growth hormone is not the primary hormone for calcium regulation in response to high blood calcium.

Answer:

D. calcitonin; more urinary phosphate reabsorption