What is the primary function of the
parathyroid gland? (A) To regulate the basal metabolic
rate of the body. (B) To regulate the calcium levels
in the blood. (C) To control the rate of
unconscious activities, for instance, heart rate and breathing rate. (D) To regulate body
temperature. Or (E) to stimulate the uptake of
glucose by the liver.
This question asks about the
parathyroid glands. To answer this question correctly,
let’s review the key facts about the structure and function of these pea-sized
The parathyroid glands are
endocrine glands. An endocrine gland is an organ
consisting of a group of specialized cells that secrete hormones directly into the
blood. The hormones, for example, insulin,
are chemical messengers which are carried around the body to their target cells,
where they bring about a response, for example, the uptake of glucose by the liver
The parathyroid glands are two
pairs of small, oval glands about the size of peas, which lie next to the thyroid
gland in the neck and work closely with it. To see how they all work together,
let’s start with the thyroid gland.
If you look at this diagram of the
cells making up the thyroid gland, you can see there are two types of secretory
cells: follicular cells and parafollicular cells. The follicular cells secrete the
thyroid hormones thyroxine and triiodothyronine. The main function of these is to
regulate the basal metabolic rate of the body, but they also help maintain healthy
skin and hair.
Calcitonin is secreted by the
parafollicular cells, also known as C cells as they secrete calcitonin. Calcitonin decreases calcium levels
in the blood by preventing release from the bones to maintain a normal, healthy
Now let’s look at the parathyroid
glands. The parathyroid glands secrete
parathyroid hormone. This hormone works together with
calcitonin to maintain calcium homeostasis in the body. When the calcium levels in the
blood are low, parathyroid hormone stimulates the breakdown of bones to release
calcium into the bloodstream.
Calcium is the main mineral in the
bones making them dense and strong. However, calcium ions are also very
important in blood clotting, muscle contraction, and the transmission of signals in
the nervous system. So levels in the blood need to be
Calcitonin and parathyroid hormone
control the levels of calcium in the blood by negative feedback as shown in this
diagram. As levels of calcium rise in the
blood, calcitonin is released and returns them to the normal level. When calcium levels drop,
parathyroid hormone is released to raise the levels back up to the norm.
Now that we have reviewed how the
thyroid gland and parathyroid gland work together, we can return to our
question. We now know that the correct answer
to the question is (B). The primary function of the
parathyroid gland is to regulate the calcium levels in the blood.