What Is Hungry Bone Syndrome?

Published on February 16, 2022

Medically reviewed by Stuart Hershman, MD

Hungry bone syndrome (HBS) is a rare condition characterized by prolonged and significant hypocalcemia (low calcium), hypophosphatemia (low phosphorus levels), and hypomagnesemia (low magnesium).

It is a serious complication of parathyroidectomy surgery (surgery to remove the parathyroid glands or parathyroid tumors) or after a thyroidectomy (surgery to remove part or all of the thyroid gland). Surgery is not the only risk factor, and HBS can be quite serious if left untreated.

This article will discuss hungry bone syndrome, its symptoms, causes, how its diagnosed and treated, and more.

Ultrasound examination of thyroid — choja / Getty Images

Hungry Bone Syndrome Symptoms

In addition to low calcium levels, people with HBS will experience hypophosphatemia and hypomagnesemia. They might also experience muscle weakness, fatigue, and vitamin D deficiency.

Additional symptoms of hungry bone syndrome are those related to severe hypocalcemia and include:

Confusion or memory loss
Muscle spasms and cramping
Numbness and tingling of the hands, feet, and face
Depression
Weak and brittle nails
Easy fracturing of bones
Bone pain

Symptoms of hypophosphatemia include:

Muscle weakness
Weak or soft bones
Muscle depletion
Altered mental state
Seizures
Numbness
Rhabdomyolysis: The breakdown of muscle tissue that leads to the release of muscle fibers into the blood, which are harmful to the kidneys

Left untreated, severe hypophosphatemia can lead to coma or death.

Symptoms of hypomagnesemia include:

Nystagmus (abnormal eye movements)
Convulsions (uncontrollable muscle contractions)
Fatigue
Muscle cramping or spasms
Muscle weakness
Numbness

Causes

Around 13% of people who undergo a parathyroidectomy for primary hyperparathyroidism (overactive parathyroid gland) will develop hungry bone syndrome. While doctors recognize this as a complication of thyroid surgery, the research on this condition is very limited.

A fall in calcium blood concentration levels is common post parathyroidectomy in people with hyperparathyroidism, but this usually resolves within days. A severe drop in calcium serum levels for more than four days becomes HBS.

HBS is reported in 25%–90% of people with radiological (X-ray) evidence of hyperparathyroid bone disease (also called osteitis fibrosa) and in up to 6% of people without skeletal involvement.

Hyperparathyroid bone disease occurs when one of the parathyroid glands is overactive (hyperparathyroidism) and there is too much parathyroid hormone (PTH), which causes bones to release too much calcium into the bloodstream. This causes the bones to lose their density and hardness.

Various risk factors are believed to contribute to the development of HBS. These might include:

Older age
Weight and volume of resected parathyroid glands
Radiological evidence of bone disease from osteitis fibrosa
Vitamin D deficiency
Primary hyperparathyroidism
Secondary hyperparathyroidism: This type of hyperparathyroidism occurs when another condition causes low calcium levels in the body, and over time, there will be increased PTH levels.
Thyrotoxicosis: This is excess thyroid hormones in the body. People with this condition also have low levels of thyroid-stimulating hormone (TSH) in the bloodstream.
Osteoblastic bone metastasis: Hungry bone syndrome has also been linked to this type of spread of cancer to the bones. It is sometimes seen in cancer of the lung, kidney, breast, and prostate. In these instances, it is the result of excessive new bone formation.
Parathyroid carcinoma: This is an extremely rare but aggressive and life-threatening type of hyperparathyroidism.

Diagnosis

HBS often goes undiagnosed after parathyroidectomy for primary hyperparathyroidism or secondary hyperparathyroidism. You should reach out to your doctor if you experience symptoms of hypocalcemia, especially following parathyroid or thyroid surgery.

People with HBS will present with signs and symptoms of hypocalcemia. Physical examination might show signs of fracture (bone breaks), bone deformities, recent surgical scarring from parathyroid or thyroid removal, and continuous muscle fiber activity with cramping and muscle relaxation.

Your doctor will check your blood calcium levels. Severe and persistent low calcium levels of less than 8.4 milligrams per deciliter (mg/dL) for more than four days after surgery, along with hypophosphatemia, are usually enough to make a diagnosis of HBS.

Low calcium levels are believed to occur after a sudden drop in PTH levels that had been high. This leads to an influx of calcium to “calcium-starved” bone.

HBS is often reported after parathyroidectomy or thyroidectomy because of primary or secondary hyperparathyroidism. The longer the PTH levels remain elevated, the greater the severity of hypocalcemia after surgery.

Treatment

Left untreated, HBS can bring out complications linked to hypocalcemia. Complications might include seizures, cardiac arrhythmias (irregular heartbeats), severe muscle spasms and cramping, and impaired brain and motor function.

The main goal of treatment for HBS is to replenish calcium. This is often done through calcium supplementation and with high doses of vitamin D and electrolytes.

The recommended treatment for HBS starts with elemental calcium between 6 and 12 grams per day. The calcium is administered intravenously (IV, within a vein) and then switched to oral supplements.

Hypomagnesemia is treated as needed. Doctors usually delay treatment for low magnesium levels because magnesium supplementation might slow the effect of calcium replacement.

Hypophosphatemia treatment is also often delayed until calcium levels have improved for similar reasons. Both magnesium and phosphorous levels can return once calcium levels are leveled.

Research shows correcting vitamin D levels can significantly reduce the effects of hungry bone syndrome. Vitamin D deficiency is often associated with hungry bone syndrome and hyperparathyroidism, and restoring vitamin D to normal levels can help balance out calcium.

Prognosis

For most people with HBS, the prognosis can be good. However, there is some variability in how long it takes for the condition to improve. In some cases, the need for calcium and vitamin D replacement can take up to one year after surgery.

For some people, hungry bone syndrome can last up to nine months before calcium levels return to normal. It may take even longer if HBS is caused by parathyroid carcinoma.

Summary

Hungry bone syndrome is a rare condition that causes prolonged and significantly low calcium levels. It might also cause decreased phosphorus levels and low magnesium. Risk factors for this condition are parathyroidectomy and thyroidectomy surgeries.

HBS is often treated by restoring calcium levels and balancing out vitamin D. The prognosis for most people with HBS is favorable, especially once calcium levels are replenished.

A Word From Verywell

If you have parathyroidectomy or thyroidectomy, you may develop postoperative hypocalcemia or hungry bone syndrome. Your healthcare team will work hard to monitor you after surgery to check calcium, parathyroid hormone, phosphorous, and magnesium levels.

If you have other risk factors, such as hyperparathyroid bone disease, you should discuss your risk of hungry bone syndrome with your doctor. This is the best way to get ahead of problems and complications of severely low calcium.

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