Parathyroid hormone and calcitonin interact in regulating calcium levels in the body. Experiments showed that parathyroid hormone opposes the effects of calcitonin, raising calcium levels when they are low. When bone and parathyroid tissues were incubated together, parathyroids raised calcium levels higher than thyroids alone. This suggests that parathyroids have two ways of affecting bone: by counteracting calcitonin and directly influencing calcium levels.