Back To CoursePathophysiology Textbook
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Jen has taught biology and related fields to students from Kindergarten to University. She has a Master's Degree in Physiology.
Your skeleton does much more than just provide a framework for your muscles and a system of protection for your organs. It also serves as a storage system for calcium, an ion integral to the proper function of every organ system in the body. Normally, the parathyroid glands play a role in calcium homeostasis by causing release of calcium from this storage system into the blood. Hyperparathyroidism - too much parathyroid hormone - causes the body's system of calcium homeostasis to fail.
Let's start by reviewing a little about the parathyroid glands. Not easily seen because they are hidden behind the much larger thyroid gland, the four parathyroid glands make and secrete parathyroid hormone, or PTH, a hormone that regulates calcium and phosphorous levels. PTH increases the level of calcium circulating in the blood.
Let's see how.
A low level of calcium signals receptors in the parathyroid glands to release PTH. PTH encourages the body to retain calcium by acting on bone, which releases calcium ions; the intestine, which reabsorbs calcium into the bloodstream; and the kidneys, which also reabsorb calcium into the bloodstream. The kidneys activate vitamin D and excrete phosphorous into the urine. As the parathyroid glands' receptors sense rising calcium levels, they signal the parathyroid glands to stop releasing PTH.
An appropriate concentration of calcium in the body is vital. We usually think of bones and teeth when we think of calcium. Although this function of calcium is important, this mineral does much more than just support our skeletal system.
First, cell communication would not be possible without calcium. Calcium plays a major role in delivering biochemical messages needed to produce hormones, neurotransmitters, and other types of cell signals. Calcium is necessary for normal blood clotting. And calcium is crucial for proper muscle contraction and relaxation. All types of muscle - cardiac, smooth, and skeletal - require calcium ions to function.
Now that we have in mind basic calcium function and regulation, let's compare the two main types of hyperparathyroidism. As we start, remember that 'hyper' means 'to excess.' For example, when we have too much caffeine, we can get hyperactive - jittery and shaky. This root and its meaning will play a part in several of the terms we're going to use.
In primary hyperparathyroidism, there is a problem with the parathyroid glands that causes them to produce too much PTH. In most cases, this is due to either an adenoma, a non-cancerous tumor of the parathyroid gland, or simple hyperplasia, an increase in the number of cells that make up the parathyroid glands. In both, at least one of the parathyroid glands becomes enlarged. Exactly why adenomas and hyperplasia occur in the parathyroid gland is still unknown, but genetics probably plays a role.
If primary hyperparathyroidism is caused by adenoma, the negative feedback system fails; that is, the receptors that normally tell the parathyroid gland to stop making PTH don't work. In cases caused by hyperplasia, the increase in the number of cells leads to an increase in the amount of PTH. The bottom line? Too much PTH is created and released.
Remembering that PTH's job is to encourage the kidneys and intestine to retain calcium and to release it from bone, it makes sense that an increase in PTH should cause high levels of calcium in the blood. Indeed, this is one of the major symptoms of primary hyperparathyroidism: hypercalcemia.
A classic way to remember specific symptoms of primary hyperparathyroidism is to remember the saying 'bones, stones, abdominal moans, and psychic groans.'
The skeletal system especially suffers from over-release of PTH. Osteopenia - literally 'deficient bones' - results when too much calcium is removed from bones and teeth. Patients experience brittle, fractured or broken bones and bone pain.
The other symptoms in the saying come from hypercalcemia.
Kidney stones are created when excess calcium clogs up the delicate nephrons of the kidney, forming crystals of calcium that condense into stones. Hypercalcemia also causes calcium to spill into the urine. Water follows this excess calcium by osmosis and creates an increase in urine volume.
What about the abdominal moans? Remember, PTH not only encourages calcium uptake from the intestine, it also affects the contraction of the smooth muscle of the digestive tract. Excess PTH causes GI symptoms like nausea, vomiting, abdominal pain, and loss of appetite.
Finally, the psychic groans of the saying stem from hypercalcemia's effect on neurotransmitters. This can cause symptoms like depression and memory problems.
Although not mentioned in the saying, don't forget that cardiac and skeletal muscle are affected, too. Slowed heart rate and increased blood pressure are common, as are tremors, muscle pain, and weakness.
Primary hyperparathyroidism is diagnosed by laboratory tests. These indicate high-circulating levels of PTH, hypercalcemia and low blood phosphorous, as well as calcium in the urine. Surgical removal of the parathyroid glands is the only cure for this type of hyperparathyroidism but is only recommended in severe cases. More moderate cases are treated with calcium supplements of 800-1000 mg per day plus appropriate amounts of vitamin D.
Secondary hyperparathyroidism also involves too much PTH. Unlike primary hyperparathyroidism, the increase is caused by overstimulation of a normal number of parathyroid cells. Surprisingly, this is usually due to chronic renal failure.
Why? In chronic renal failure, a reduced ability to filter blood makes the kidneys unable to recover enough calcium into the blood and to excrete enough phosphorous into the urine. This low concentration of calcium and high concentration of phosphorous are recognized by receptors in the parathyroid gland and set off the PTH pathway we talked about earlier in the lesson.
Chronic vitamin D deficiency can also cause secondary hyperparathyroidism. A low level of vitamin D may prevent sufficient absorption of calcium in the intestine. This results in low blood calcium and increased PTH release in an effort to bring calcium levels to normal. Healthy kidneys are also necessary to activate vitamin D, so renal failure can make an already existing problem even worse.
Notice that the cause of secondary hyperparathyroidism is going to lead to a difference in its symptoms. Broken, brittle, and fractured bones are still going to be a problem. However, since hypercalcemia is not present, you don't see the stones, moans, and psychic groans that you do in primary hyperparathyroidism.
Like primary hyperparathyroidism, secondary hyperparathyroidism is diagnosed by blood tests. Also like in primary hyperparathyroidism, these indicate high PTH but also show either low or normal concentrations of calcium. They also usually indicate high phosphate in blood because of poor kidney function. The exception is if the disease is caused by vitamin D deficiency. In these cases, phosphorous is usually low because the kidneys are healthy and excrete enough phosphorous into the urine.
Treatment for secondary hyperparathyroidism depends on its cause. In cases of vitamin D deficiency, the treatment is simple: a mega dose of vitamin D, 50,000-IU once a week for eight weeks, with a second round of eight weeks if necessary.
If the condition is caused by chronic renal failure, treatment must be tailored to the needs of the patients. In severe cases, surgical removal of the parathyroid glands is an option. However, most cases are treated with a low-phosphorous diet coupled with vitamin D and calcium supplements.
Let's finish by comparing primary and secondary hyperparathyroidism.
Primary hyperparathyroidism is caused by an increase in the number of cells in the parathyroid glands. Secondary hyperparathyroidism is caused by too much stimulation of a normal parathyroid gland. Both forms cause the release of too much parathyroid hormone, or PTH. Too much PTH means that the body's system of calcium regulation will fail.
In primary hyperparathyroidism, this failure means hypercalcemia (too much calcium in the blood). Hypercalcemia is responsible for the bones, stones, abdominal moans, and psychic groans of primary hyperparathyroidism.
The root cause of secondary hyperparathyroidism is usually either chronic renal failure or vitamin D deficiency. Since these prevent proper calcium uptake by the intestine and kidney, there's no hypercalcemia, and you don't see the range of symptoms you do in primary hyperparathyroidism.
Both forms are diagnosed by blood tests. Both show high PTH, but primary hyperparathyroidism indicates high calcium, and secondary hyperparathyroidism shows normal circulating calcium but high phosphorous. Both are treated with either surgery or, more commonly, vitamin D and calcium supplements.
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Back To CoursePathophysiology Textbook
20 chapters | 274 lessons