Module 2 Lesson 4 - 3
Lesson 4 — The Thyroid and Parathyroid Glands
Antagonistic Hormones Regulate Calcium Levels
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Two antagonistic hormones, which are hormones that work in opposition to one another to establish overall balance or homeostasis, regulate calcium levels in the blood. One of these hormones, calcitonin, originates from the thyroid gland, and the other, parathyroid hormone (PTH), from the parathyroid glands.
Calcium is a critical element in the body. Calcium is basis for bones in skeleton and tooth development. It is essential for blood clotting process, synaptic transmission in nerve conduction, and muscle contraction.
Calcitonin
Calcium homeostasis in the blood is regulated by calcitonin, produced by the thyroid gland, and parathyroid hormone (PTH), produced by the parathyroid glands. These two hormones work opposite of each other: calcitonin decreases levels of calcium in the blood, and parathyroid hormone increases levels of calcium in the blood.
Calcitonin decreases levels of blood calcium by stimulating uptake of calcium into the bones and by inhibiting certain bone cells from breaking down bone in the skeleton to release calcium.
High levels of calcium in the blood stimulate the secretion of calcitonin, which increases the uptake of calcium into the bones and inhibits the decomposition of bone, thereby lowering calcium levels in the blood. Low calcium levels in the blood inhibit the secretion of calcitonin, which decreases the uptake of calcium into the bones and allows the decomposition of bone to release more calcium into the blood.
A negative feedback loop regulating blood calcium levels by calcitonin resembles the following diagram:
Parathyroid Hormone (PTH)
The effect of parathyroid hormone (PTH) is to increase levels of calcium in the blood plasma. It does this by
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stimulating bone decomposing cells to digest bone in the skeleton and release the calcium from this decomposed bone into the bloodstream
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stimulating the kidneys to reabsorb more calcium, thereby decreasing the amount of calcium excreted in the urine, which consequently increases the levels of calcium in the blood
- stimulating cells in the intestines to increase the absorption of calcium from food, thereby increasing the levels of calcium in the blood
Vitamin D is required for the absorption of calcium from the intestines, and parathyroid hormone stimulates the activation of Vitamin D to accomplish this. For this reason, vitamin D is added to dairy products frequently.
The negative feedback loop for the regulation of blood calcium levels by parathyroid hormone resembles the following:
