Health, advice, and information online community for dog and cats lovers.

Hyperthyroidism cats – old, skinny, and hungry

Hyperthyroidism Cats“My cat seems to be losing weight no matter how much I feed him.”

“My cat can’t seem to get enough to eat or drink!”

“Why is my cat losing weight? I keep feeding her more but it doesn’t help – she still yowls a lot.”

These are how many of my appointments with hyperthyroid cats start. But they always end with a pet owner relieved to learn that a wide variety of treatment options are out there. It’s a straightforward, treatable disease, especially when detected early. Here’s what you need to know.

The thyroid gland controls the body’s metabolism. Whenever I put on a few pounds, I often wish I could stimulate my thyroid for a while to burn the extra fat, but alas, we cannot. An over-active thyroid gland (hyPERthyroid) causes the body’s metabolism to elevate, while an under-active thyroid gland (hyPOthyroid) causes the metabolism to lower, leading to lethargy and weight gain.

Cats typically do not develop hyPOthyroidism…as much as the owners of fat cats wish they could blame that! HyPERthyroidism, however, is quite common in cats. The excess thyroid hormone causes the body to burn calories very quickly. No matter how much these cats eat, they continue to lose weight. They seemed amped up on caffeine. Some owners even notice their hearts racing. Other systems are affected as well, though. That’s why simply raising a thyroid level is not a good weight loss plan.

Symptoms of hyperthyroidism can include increased appetite, weight loss, increased thirst, decreased desire to sleep, and increased vocalization. Many cats develop a poor, greasy, flakey haircoat as well.

The blood pressure rises, sometimes to dangerous levels. These cats are often restless because of it. Many people don’t see this symptom until it becomes severe enough that the cat becomes suddenly blind. This (thankfully) only happens in a small number of hyperthyroid cats – often before they are diagnosed and treated. The very high blood pressure (hypertension) causes the retina, the thin layer of cells on the back of the eye, to detach. The retina is responsible for processing light and sending the signal to the brain, which interprets it into what the cat sees. When the brain is not getting a signal, it figures there must not be enough light, so it tells the pupils to open up (dilate) as wide as they can. These cats often present with “his eyes look funny” as the complaint. Those poor pupils are as wide as they can be, but the cat still isn’t seeing anything. Not all hyperthyroid cats develop hypertension severe enough to cause retinal detachment. Hopefully we catch it and treat it before the irreversible damage is done.

Another organ affected by the high blood pressure that comes for the overactive thyroid is the kidneys. Old cats typically don’t have award-winning kidney function anyways, but the increased blood pressure can make them look better than they actually are. With the blood pressure being high, all this blood is flowing through the kidneys, who are thinking “this is great!” Kidneys that are beginning to fail can “fake it” with the increased blood pressure. When your veterinarian runs blood tests, the kidney levels may look slightly elevated, or even normal. However, once we get the thyroid level, and therefore the blood pressure, under control, the kidneys’ true colors emerge. Your veterinarian may say the the kidney levels are elevate, and the kidney function is insufficient (hate to use the word “failure” until it’s severe).  Did treating the thyroid and lowering the blood pressure cause the kidney disease? No, it just pulled back the curtain that the kidneys were hiding behind, so now we can see what they really are doing. The loss of function was there before, we just couldn’t see it. Even though the high blood pressure makes the kidney levels appear better than they should be on blood tests, the kidneys are working harder, and will not function as well in the long run. Avoid the temptation to keep the blood pressure high just to make the kidneys “look” good.

Yet another organ system affected by hyperthyroidism is the heart. This should come as no surprise, given the high blood pressure aspect of the disease. In addition, cats develop a specific type of heart disease called HCM: Hypertrophic CardioMyopathy. In this disease, the walls of the heart thicken, or hypertrophy, just like any other muscle grows when an increased work load is placed on it. This growth in the heart is very bad, because the walls of the ventricles grow inward. Picture a glass jar that is empty, and how much water you can put in it. Now picture the jar, but add to the wall thickness on the inside. What happens? Not nearly as much water fits into the jar! That is what happens with these cats’ hearts. The walls become thick, and not very much blood can fit into the chambers. This is obviously a bad thing when the heart cannot pump the volume of blood it used to.

To boot, a complicating factor that can stem from this is the formation of blood clots. One scenario that we see most often is called a “saddle thrombus.” Yes, a blood clot can go anywhere is wants to, but a common route is to start in the heart and travel down the aorta, the main blood vessel going down the body. When the aorta reaches the legs, it splits into two arteries, one for each leg. The clot can’t make the split, so is lodged in the “Y”, blocking circulation to the back legs. This is incredibly painful, as you can imagine. We used to euthanize these poor cats, because treatment options didn’t really exist. Fortunately, clot-busting treatments are becoming more and more available, thanks to human medicine.

So while having an overactive thyroid may sound quite appealing from a weight-loss perspective, it’s actually much more involved. The good news is there are a wide variety of treatment options out there! My next article will spell out the options.

Related: Treatment Options For Hyperthyroidism Cats

Web-DVM guest blogger Dr. Karen Louis is a practicing small animal veterinarian.  See more of her articles at her blog at

One thought on “Hyperthyroidism cats – old, skinny, and hungry

  1. To boot, a complicating factor that can stem from this is the formation of blood clots. One scenario that we see most often is called a “saddle thrombus.
    Thanks for sharing!

Leave a Reply

Your email address will not be published. Required fields are marked *


The Space Coast Pet Podcast


Read Dr. Roger’s Latest Book!

The Man In The White Coat: A Veterinarian's Tail Of Love