Calcitonin stimulates the production of active vitamin D and the absorption of calcium in the intestines. The researchers studied rats without thyroid and parathyroid glands to see how calcitonin affects these processes. They found that calcitonin infusion increased levels of active vitamin D in the blood, especially when rats were on a diet without calcium. This increase in active vitamin D led to higher levels of calcium in the blood, mainly by enhancing the absorption of calcium in the intestines. Overall, the results suggest that calcitonin and calcium independently regulate the production of active vitamin D, and calcitonin specifically boosts the absorption of calcium in the body.