Problems with the parathyroids: what is hyperparathyroidism in cats and dogs? 

Hyperparathyroidism – a condition you may not have come across before and one that’s a bit of a mouthful to say! If your dog or cat has just been diagnosed with hyperparathyroidism, then you may have a few questions.  

Most people have heard of the thyroid glands, but they are less familiar with the parathyroids. These smaller glands still have a very important job to do, helping to keep calcium levels in the body steady.  

So when these go wrong, we can start to see problems.  

What is hyperparathyroidism? 

Let’s start with some anatomy! The parathyroid glands are four small glands, two found on either side of the neck. These parathyroids are attached or embedded in the larger thyroid glands, of which there are two, one on either side of the neck. The parathyroid glands are only approximately 2-3 mm in length, but they perform a big job. They help to regulate calcium levels in the blood by producing parathyroid hormone.  

Calcium is extremely important in the body as it helps to control the way that all muscles and organs (including the brain, kidneys, and bowels) function. It is particularly essential for muscle contraction and stabilises nerve cell membranes. 

In cases of hyperparathyroidism, these glands are producing too much parathyroid hormone, and calcium levels around the body start to rise. If levels of calcium are not appropriately maintained and controlled, then the animal can start to become unwell. 

It is worth noting that this condition is different from hypoparathyroidism which is where parathyroid hormone levels are low, and calcium levels in the body decrease.  

What causes hyperparathyroidism? 

There are two types of hyperparathyroidism, primary and secondary. These have different underlying causes – 

Primary hyperparathyroidism (PHPT) 

Primary hyperparathyroidism (PHPT) is a condition that is rare but can be found in some older dogs and cats, because of a benign change to one or more of the parathyroid glands. This can be due to a single adenomatous growth or more generalised parathyroid hyperplasia. Either way, enlargement of the affected gland occurs, leading to more hormone production. This condition can affect any breed but is common in Keeshonds.  

In around 5% of cases hyperparathyroidism can be caused by a malignant tumour called a parathyroid carcinoma.  

The reason why PHPT occurs is unknown. 

Secondary hyperparathyroidism  

Secondary hyperparathyroidism is more common than primary and can occur due to – 

Malnutrition –  

Too little calcium and vitamin D in the diet, or an excess of phosphorus. This is most likely with badly formulated diets or diets that have been home-cooked without the advice of a veterinary nutritionist. The body releases excess parathyroid hormone which leads to bones in the body becoming demineralised to raise calcium levels in the blood. The right balance of calcium and phosphorus is especially important in young, growing dogs which is why feeding an age-appropriate diet is essential.  

Chronic kidney disease –  

Badly diseased kidneys cannot excrete enough phosphorus causing a buildup of phosphate in the blood. Blood calcium binds to this phosphate and deposits in body tissues, where it is no longer useful. The kidneys are also responsible for producing calcitriol, a form of vitamin D which promotes the absorption of calcium from the intestines and helps bones to resorb calcium. These two things both cause the parathyroid glands to release more hormone to try and raise calcium levels. 

Cats with hyperthyroidism –  

There is an association between cats with hyperthyroidism and secondary hyperparathyroidism. This is thought to be due to the effects that increased thyroid hormones have on the kidneys and bone turnover. 

Dogs with hyperadrenocorticism –  

Dogs with hyperadrenocorticism (Cushing’s disease) produce increased amounts of cortisol. There is an association with Cushingoid dogs and hyperparathyroidism, leading them to suffer from altered calcium metabolism. Calcium deposits such as calcinosis cutis in the skin can occur. 

And just to complicate things…  

we can also see pseudohyperparathyroidism!  In this condition we see calcium levels increase because of malignant cancer such as lymphoma or dogs with anal gland tumours. These tumours release a protein that has a similar structure to parathyroid hormone, causing calcium levels to rise. This carries a poor prognosis.  

What are the symptoms of hyperparathyroidism? 

Symptoms of hyperparathyroidism can be vague, and some animals are completely asymptomatic. An issue may just be picked up after a routine blood test shows an elevation in blood calcium.  

Possible symptoms could include – 

How is hyperparathyroidism diagnosed? 

Your vet will start by examining your pet and discussing their clinical history with you, including their normal diet and any symptoms. They will check your pet’s clinical notes for details on other health conditions. Your vet will give your pet a nose-to-tail examination.  

Blood tests are usually advised next 

At this point, a general health screen is often advised, especially if symptoms are vague. Your vet will look at your pet’s calcium levels, but they will also want to check their organ function, looking at their kidney and liver parameters. Phosphorus levels might also be decreased in some affected animals, but not all. They may also check your pet’s haematology (red and white blood cells) to rule out other disease processes.  

It is often necessary to do more specific blood tests to reach a definitive diagnosis 

This may include an ionised calcium blood test which looks at biologically active calcium in the blood and is preferred to looking at your pet’s total calcium alone. Your vet may need to rule out secondary hyperparathyroidism causes first, such as kidney disease or Cushing’s disease, as these are much more common than primary hyperparathyroidism. 

If your vet still suspects a primary hyperparathyroidism problem, they will suggest a blood test that measures your pet’s parathyroid hormone levels (PTH). Some labs will also offer measurements for parathyroid hormone-related protein (PTHrP) which may be helpful when trying to rule out a malignant cancer as a cause.  

If both PTH and calcium levels are consistently high, then a diagnosis of hyperparathyroidism is often made.  

In some cases, diagnostic imaging may also be recommended by your vet. Ultrasound scans of the parathyroids can help to identify enlargement and see which glands are specifically affected, which can help with treatment.   

How is hyperparathyroidism treated? 

Treatment will depend on which type of hyperparathyroidism your pet has.  

Primary hyperparathyroidism  

If left untreated, long-term high calcium levels can cause calcium deposits in body organs leading to serious issues including kidney damage. So even if a dog is asymptomatic, treatment is advised.  

Surgical removal of the affected parathyroid gland is recommended (parathyroidectomy). Ultrasound scanning before surgery may help to determine which gland needs removing, but in some cases, the correct gland can only be identified during the operation. This surgery is challenging, due to the size of the glands and the risk of causing issues with calcium levels in the body. A sudden drop in blood calcium may occur shortly after surgery, which can cause issues like seizures. Most pets are hospitalised for monitoring after surgery for this reason.  

The prognosis for dogs that are successfully treated is generally excellent. A small percentage may require repeat surgery if any of their remaining parathyroid glands become affected in the future.   

Because of the nature of the surgery, this is usually carried out in a referral-level hospital. It is certainly not a condition that vets in general practice treat every day. 

Secondary hyperparathyroidism  

This usually involves managing the underlying disease process which your vet will discuss with you. Malnutrition cases can be straightforward, depending on how advanced their symptoms are, but other illnesses may be a bit trickier. 

For example, chronic kidney disease is progressive, and kidney function will deteriorate into failure over time. Changes in diet to reduce phosphorus levels or administering phosphate binders can be helpful to control secondary hyperparathyroidism and improve lifespan. This reduces the amount of phosphorus present which in turn reduces the amount of calcium-phosphorus deposits around the body. Calcitriol supplementation may also be needed.  

You must speak to your vet if you have any questions regarding your pet’s condition, including costs and treatment options.   

Summary 

Hyperparathyroidism is a condition that affects calcium levels in the body which if left untreated, could cause some very serious issues for your pet. Primary hyperparathyroidism is less common than secondary causes of the disease. It is important for your vet to diagnose the type of hyperparathyroidism so that they can work out the best course of treatment for them. 

Hopefully, we’ve helped to answer a few questions, but you shouldn’t hesitate to speak to your own vet who will be able to give you advice more specific to your dog or cat.