Your Horse’s Thyroid Gland

The horse’s thyroid gland has been the subject of a lot of discussion over the course of my career.  It’s still the subject of a lot of attention – I recall reading a few years ago that something along the lines of ¾ of a million pounds of supplemental thyroid hormone is sold to veterinarians each and every year. That’s a lot of thyroid hormone.  In fact, it’s kind of alarming.  So what’s going on?

WHAT’S A THYROID GLAND?

The horse’s thyroid gland is in the upper part of the horse’s neck, right in the throat latch area.  You can often feel it – I’ve even gotten calls from concerns clients who felt the thyroid gland and were concerned it was some sort of a tumor.  It sits astride the horse’s windpipe (trachea) almost like an English saddle.  Hormones* released by the horse’s thyroid gland run through his bloodstream and affect almost every part of his body, although, to be sure, we don’t know exactly what the hormones do to every part if his body.  We do know that the substances released from the horse’s thyroid gland help control the horse’s metabolism, that is, how the horse’s body’s cells use the energy obtained from feed. Important stuff.

WHAT’S THE PROBLEM?

In several species, including cats, dogs, and yes, people, it’s not uncommon for the thyroid to have problems with not being active enough.  An underactive thyroid gland is what’s referred to by the term, “hypothyroidism.”  In hypothyroidism, the thyroid gland doesn’t make enough of the thyroid hormones, and body metabolism can be affected, as a result.

HOW DO YOU TELL IF A THYROID GLAND ISN’T ACTIVE ENOUGH?

Well, to be honest, for horses (and horse owners) here’s where some of the problems start.

In the other species, typical clinical signs lead one to believe that the thyroid gland is a problem.  The lack of thyroid hormone affects body processes.  People with hypothyroidism report a lack of energy, and their metabolism slows down.  Hypothyroidism causes a lot of vague clinical signs.  They’re vague in the sense that people usually can’t immediately tell exactly what the problem is:  things such as depression, hair loss, fatigue, reproductive problems, problems with weight gain, or difficulty losing weight.  Further making the diagnosis difficult is that problems tend to develop slowly, over years.  But with horses, you can throw all of the above out of the window.**

HOW DO YOU MAKE A DIAGNOSIS OF THYROID DISEASE?

In other species, a diagnosis of hypothyroidism is fairly straight forward most of the time.  Thyroid hormone is easy to measure in a blood test.  The combination of clinical signs and low blood levels of thyroid hormone are pretty suggestive of disease of the thyroid gland.

SOUND LIKE A HORSE YOU KNOW?

Probably.  Which is part of the problem.

YOU SEE, IT GETS COMPLICATED

When I got out of school, it seems I ran into horses that were said to be “hypothyroid” all of the time (still do, actually).  These horses tended to be overweight – they were “easy keepers.”  Perhaps they were a little bit lethargic:  not wanting to jump with the appropriate amount enthusiasm, for example.  Or you’d have a mare that was having troubles getting in foal.

You’d draw a blood sample and low and behold, the circulating levels of thyroid hormone would be below the normal ranges given to you by the laboratory.  Bingo!  You’d have an easy diagnosis, a treatable problem (give more thyroid hormone) and, in fact, when on thyroid hormone – along with dietary adjustments – the horses would tend to lose weight.  Or maybe the mare would get pregnant. Everyone was happy.

Except…  it turned out – as it does so many times in medicine – that things were not that easy.

The first fly in this ointment is that we know what horses look like when they are hypothyroid.  I mean, we know what they look like when they are really hypothyroid.  That is, we know what horses that are so hypothyroid that they don’t even have a thyroid gland looks like (because it got removed surgically).  This actually happened a few decades ago, and the horses that had their thyroid glands removed didn’t get fat.  Their hair coats got dull.  The horses got skinny at first, but even that didn’t last; believe it or not, at least of those horses lived to well into her 30’s, and she was as big as a house.  Otherwise, stated, without a thyroid gland – completely hypothyroid, as it were – horses seem to do pretty well.  The also reproduce normally, without any problems, when they don’t have a thyroid gland, as well.  In other words, horses without thyroid glands look pretty much nothing like the horses that usually get commonly diagnosed as having thyroid gland problems by concerned onlookers.

BUT WHAT ABOUT THE BLOOD TESTS?

Right.  Well, it turns out that you can’t make an accurate diagnosis of thyroid disease in a horse by running a single blood test.  That’s because the levels of thyroid hormone in a horse’s body go up and down like a seismograph.  Depending on when you draw blood, levels of thyroid hormone can be above or below the lab normal:  you never know.

Here’s another thing.  Just because a test can be run by a lab doesn’t mean that its interpretation is necessarily valid. In fact, you have to be a bit careful here, because some of the labs that are responsible for the over-diagnosis of hypothyroidism in healthy horses also sell their owners thyroid supplementation products – the fox guarding the thyroid glandular hen house, as it were.  And many non-thyroid-related illnesses can cause alterations in thyroid hormone levels, too.

So with thyroid blood tests, it’s can be a bit of a crap shoot. Nevertheless, what we do know is that running a blood test and testing for levels of thyroid hormone is not a good way to diagnose thyroid disease in the horse.

WHAT ABOUT THOSE FAT, LETHARGIC, EASY KEEPERS?

Right.  They’re still around.  A good number of them are fat and lethargic because they get too much to eat.  They came up with the phrase, “Eating like a horse,” for a reason, and as it turns out, if you keep overfeeding a horse, he’ll get fat.  Works that way with people and dogs, too.  You know this.  And if not, here’s an article for you.

But fatness in horses is a bit more complicated than just too much to eat, and there’s a whole fairly-well-misunderstood but very real phenomenon called “insulin resistance” or “metabolic syndrome” that has internal medicine and endocrinology folks all in a twitter, as well as horse owners, by proxy.  These horses tend to gain weight easily, put down unusual amounts of fat in unusual places, develop fun conditions like laminitis, and, in short, are “easy keepers” for a reason.  However, that reason is not a lack of thyroid hormone.

AND HERE’S WHERE IT CAN GET CONFUSING, THYROIDWISE

Remember your horse’s metabolism?  If you give thyroid hormone to a horse – whether his metabolic functions are perfectly normal or not – you’re going to increase his metabolism.  You’ll speed up the rate at which he burns calories.  Most likely, he’ll lose weight.

If your horse has insulin resistance, there’s good news, too.  Supplemental thyroid hormone helps insulin do its job  – at least for a while – and it’s very common to prescribe thyroid hormone to horses that have been diagnosed with insulin resistance.  So maybe, all those years ago, when we ran into fat, lethargic horses, and mistakenly made a diagnosis of hypothyroidism, we were actually helping the horse by giving him thyroid hormone.  We were doing the right thing for the wrong reason, as it were.

But giving thyroid hormone to a horse is not necessarily without consequence.  In other species, too much thyroid hormone when it’s not needed is definitely bad for a body. Thyroid hormone supplementation to address obesity in people ran in to a problem with increased risk of cardiac arrhythmias (for example, atrial fibrillation), so most doctors don’t recommend it.  Too much thyroid hormone can cause things like nervousness, excessive weight loss, and muscle weakness, too.  These things aren’t routinely reported in horses getting thyroid medication, but that’s probably because the horses aren’t getting a big overdose (otherwise stated, horses are big, and it can be hard to overdose them).

SO WHAT’S THE BOTTOM LINE HERE?

If your horse has been diagnosed as being hypothyroid, it’s a diagnosis that’s most likely been made in error.  True hypothyroidism is essentially unknown in horses, and a simple blood test isn’t diagnostic for thyroid problems.  If your horse has insulin resistance/metabolic syndrome, thyroid hormone is likely to be of some help in the short term, but the key to management is proper dietary control.  If you’ve got a mare, and you want to get her in foal, there’s very little reason to use thyroid hormone at all.

As it turns out, your horse is extremely unlikely to have a true problem with his thyroid gland, but there are still reasons to use thyroid hormone.  But there’s never any reason to use it willy-nilly, and horses shouldn’t need to be on the stuff long-term.  Fortunately, based on the amount of thyroid hormone that’s sold for what-would-seem-to-be-somewhat curious reasons, there doesn’t seem to be a lot of problems that come from giving horses extra thyroid hormone, but I hope you’d agree that there’s no point in giving a horse medicine if he doesn’t need medicine.

When it comes to your horse’s thyroid gland, it’s complicated.  Medicine often is.  But if you’re wondering if your horse is hypothyroid, he’s almost undoubtedly not.

Thanks to my friend Dr. Philip Johnson, equine endocrinology expert and Professor of Equine Medicine at the University of MIssouri College of Veterinary Medicine, for taking the time to look over this article before I published it and make sure that I didn’t say anything I would regret later, medicine-wise.

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* Hormones are products of cells that circulates in body fluids, especially in the bloodstream.  They usually stimulate other cells to make something.  Those other cells are usually in different parts of the body fro where the hormones are produced.

** One of my favorite words in the English language is, “Defenestrate.”  It means, “A throwing of a person or thing out of a window.”  Try that one out in a conversation!