If I had to identify the single condition most often incorrectly diagnosed in herding breeds — both over-diagnosed where it isn’t present and missed where it is — I’d say hypothyroidism without much hesitation.
The thyroid hormone system is genuinely complicated. The testing is not as straightforward as many owners, and frankly many vets, assume. And the clinical signs of hypothyroidism overlap with what feels like most of the rest of veterinary medicine.
Let me try to cut through the confusion.
What the Thyroid Does
The thyroid gland sits in the neck on either side of the trachea. It produces two main hormones: thyroxine (T4) and triiodothyronine (T3). These hormones regulate metabolic rate in nearly every cell in the body. When thyroid hormone levels are adequate, cells function at the right rate. When they’re low, everything slows down.
The thyroid is regulated by the pituitary gland, which produces thyroid stimulating hormone (TSH). TSH levels rise when thyroid hormone levels fall, stimulating the thyroid to produce more. This feedback loop keeps thyroid hormone levels within a fairly narrow range under normal circumstances.
Hypothyroidism means the thyroid isn’t producing enough hormone. In dogs, the most common cause is lymphocytic thyroiditis — an immune-mediated destruction of the thyroid tissue — or idiopathic thyroid atrophy, where the gland simply gradually fails. Both result in declining hormone production over time.
The Classic Presentation
In textbook hypothyroidism, the clinical picture includes:
Weight gain without increased food intake. The metabolic rate slows; the same caloric intake that maintained weight before now produces excess. A dog that’s getting heavier despite unchanged feeding is a thyroid flag.
Lethargy and exercise intolerance. Not just “slowing down with age” — genuine, notable reduction in energy. The dog that used to race across the field at full speed now trots slowly and wants to go home.
Coat changes. The classic changes are bilaterally symmetrical hair loss — the same pattern on both sides, typically starting over the tail base, flanks, and trunk. The remaining hair may be dull, brittle, or easily epilated. The skin beneath often becomes thickened and slightly darker.
Cold intolerance. Hypothyroid dogs often seek warmth, shiver in temperatures that would previously have been comfortable, and seem to suffer more in winter.
Slow heart rate. The metabolic slowing affects the heart. Bradycardia — a slower than normal resting heart rate — is a clinical sign in some hypothyroid dogs.
Peripheral neuropathy. In some cases, thyroid disease contributes to weakness, abnormal gait, and nerve-related problems that can be confused with orthopaedic disease.
Reproductive problems. Failure to cycle, prolonged anoestrus, or abnormal cycles in bitches; reduced libido and poor semen quality in males.
The challenge: most of these signs are non-specific. Weight gain, lethargy, and coat changes describe half the dogs that come through my door with any number of different conditions. The breed predisposition in herding dogs means the diagnosis is genuinely more likely — but it also means it gets blamed for things it isn’t causing.
Breed Predisposition in Herding Dogs
Hypothyroidism is genuinely more common in some herding breeds than in others, and the rates are worth knowing.
German Shepherds have elevated rates of lymphocytic thyroiditis and hypothyroidism compared to many other breeds. I see it regularly in practice and it’s on my differential list for every GSD with appropriate signs.
Golden Retrievers — not a herding breed but closely associated in owner demographics and often mentioned in the same health conversations — have particularly high rates. Some studies suggest Goldens are among the highest-risk breeds for autoimmune thyroiditis.
The evidence for Border Collies is less dramatic, but I’ve tested and diagnosed enough hypothyroid Border Collies over the years to keep it on the list.
The Orthopedic Foundation for Animals (OFA) maintains a thyroid registry in the United States where breeders can submit results; the pattern across submissions suggests herding breed lines vary significantly in their rates.
The Testing Problem
Here’s where I want to spend significant time, because this is where hypothyroidism management most often goes wrong.
The default test for thyroid function is a serum total T4 (TT4) measurement. It’s cheap, widely available, and it’s often reported as sufficient. It isn’t.
Total T4 measures the total amount of thyroid hormone in the blood, including both the protein-bound form (biologically inactive) and the free form (biologically active). Many conditions, medications, and even physiological states can suppress TT4 without there being any actual thyroid disease. This is called the euthyroid sick syndrome or non-thyroidal illness effect — a dog can have a low TT4 purely because it’s unwell for another reason, is on certain medications, or has recently been fasted.
Commonly suppressed TT4 in dogs without true hypothyroidism:
- Any significant concurrent illness (really, almost any)
- NSAIDs, particularly at high doses
- Glucocorticoids (steroids)
- Phenobarbital (used for epilepsy)
- Sulphonamide antibiotics
- Fasting or caloric restriction
Now consider the herding breed population. German Shepherds often present with multiple health concerns simultaneously. A dog being assessed for lethargy and weight gain might be on long-term NSAIDs for hip arthritis, or on phenobarbital for epilepsy, or be otherwise unwell. The TT4 in these dogs may be suppressed by the drug or the illness rather than actual thyroid failure. Treating them with thyroid hormone replacement based on a suppressed TT4 alone is treating a false result.
The appropriate diagnostic panel for suspected hypothyroidism includes:
Free T4 by equilibrium dialysis. This measures the biologically active form of thyroid hormone and is more resistant to the non-thyroidal illness effect. It’s the most reliable single thyroid measurement available.
TSH. A dog with true primary hypothyroidism should have elevated TSH, as the pituitary is trying harder to stimulate a failing thyroid. Unfortunately, canine TSH tests are not as sensitive as we’d like — some genuinely hypothyroid dogs have TSH levels within the reference range. A clearly elevated TSH in combination with low free T4 is strong evidence for hypothyroidism.
Thyroglobulin autoantibodies (TgAA). Elevated in dogs with autoimmune thyroiditis, sometimes before clinical hypothyroidism is established. Useful for breeding dogs and for understanding the aetiology in diagnosed cases.
The combination of free T4 by equilibrium dialysis, TSH, and clinical signs is far more reliable than TT4 alone. The cost difference is not enormous and the clinical difference in correct versus incorrect diagnosis can be significant.
What Happens When You Get the Diagnosis Right
True hypothyroidism responds well to treatment. Supplementation with synthetic thyroxine (levothyroxine) is straightforward and most dogs respond within weeks. Coat improves, energy returns, weight drops to appropriate levels.
Dosing requires some calibration — the initial dose is based on body weight, but individual variation means rechecking T4 levels 4–6 weeks after starting treatment and adjusting accordingly. A dog that’s overtreated becomes hypermetabolic, anxious, and loses weight excessively; underdosing means inadequate clinical response. Getting the dose right takes one or two adjustments in most cases.
Once stable, I recheck thyroid levels every six to twelve months depending on the individual. The dose requirement can change over time, particularly in dogs with progressive thyroiditis where further thyroid tissue is lost.
The Over-Diagnosis Problem
I said at the start that hypothyroidism is both missed and over-diagnosed. Let me address the over-diagnosis side.
“My dog is tired and fat, could it be his thyroid?” is one of the commonest questions I hear. The answer is: possibly, but probably not. In the UK dog population, genuine hypothyroidism is not that common. Most overweight, lethargic dogs are overweight and lethargic because they’re eating too much and not exercised enough. Thyroid disease is the interesting diagnosis; caloric excess is the boring one. The boring diagnosis is usually right.
I have seen dogs on thyroid supplementation for years based on a suppressed TT4 that was almost certainly due to concurrent medication or illness, not true thyroid failure. When I’ve stopped the supplementation and repeated a proper panel — free T4, TSH — results were normal. The dog was not hypothyroid. It was on a medication it didn’t need.
Giving levothyroxine to a dog with normal thyroid function doesn’t make them better. It may cause them harm through overreplacement.
The appropriate work-up matters.
Thyroid and the Older Herding Breed
The connection between thyroid disease and ageing in herding breeds is one I find clinically interesting. Hypothyroidism tends to develop in middle-aged to older dogs. The typical age at diagnosis is five to eight years for most breeds.
In my experience, subclinical thyroid dysfunction — levels that are declining but not yet clearly in the hypothyroid range — is reasonably common in older herding breeds. It may contribute to the coat changes, weight gain, and reduced energy that often get attributed to “just getting older.” This is why, in my practice, thyroid function is part of the blood panel I run on dogs entering the senior window, alongside kidney and liver function.
The overlap with other age-related conditions is significant. The metabolic slowing of hypothyroidism interacts with the reduced exercise tolerance of joint disease; the weight gain of hypothyroidism worsens arthritis; the coat changes can look like, or be compounded by, seasonal allergies and skin conditions that are also common in herding breeds. An older German Shepherd with concurrent hypothyroidism and skin allergy and hip arthritis is managing several intersecting problems, and getting the diagnosis right on each one is the foundation of managing them effectively.
I’ve discussed some of this in my broader piece on senior herding breed health — the metabolic shifts that accompany ageing in these dogs, and why checking thyroid function annually becomes increasingly worthwhile as herding dogs move into their later years.
The Practical Bottom Line
If your herding breed is gaining weight despite appropriate feeding, has a dull or patchy coat, is less energetic than their usual self, or seems cold — ask your vet about a thyroid panel. Not a TT4 alone. A proper panel including free T4 by equilibrium dialysis.
If your vet is confident in the diagnosis and treatment helps, excellent. If you’re not seeing the expected improvement, question whether the diagnosis is correct and consider referral to an internal medicine specialist for a more thorough work-up.
Hypothyroidism, properly diagnosed and treated, is one of the most satisfying diagnoses in small animal medicine. The dog that was dull, stiff, and overweight transforms over six to eight weeks into something much closer to its former self.
Just make sure that’s actually what the dog has before treating for it.