What are the signs of hypothyroidism in dogs?

The classic cluster is weight gain without an increase in food, lethargy and reduced tolerance for exercise, a dull thinning coat with symmetrical hair loss over the flanks, a 'rat tail' as hair is lost from the tail, darkened and thickened skin, seeking warmth, and recurrent ear or skin infections. Any two or three of these together in a middle-aged dog is worth a blood test.

How is canine hypothyroidism diagnosed?

Through bloodwork, and it is more nuanced than a single number. Diagnosis rests on clinical and historical findings suggestive of the disease supported by laboratory results, usually total T4 with free T4 and TSH, because other illnesses can suppress thyroid readings in a dog whose thyroid is actually fine. That phenomenon is why a low T4 alone should not start lifelong treatment.

Is hypothyroidism in dogs treatable?

Yes, and it is among the more rewarding diagnoses in veterinary medicine. Treatment is daily oral levothyroxine, it is inexpensive, and most dogs improve substantially. Energy typically returns within a few weeks; coat and skin changes take longer, often two to four months. It is lifelong, and the dose needs periodic monitoring by blood test.

Can hypothyroidism cause behaviour changes in dogs?

It can contribute. Lethargy and reduced tolerance are well recognised, and some dogs become less willing to engage or more irritable, which is easy to read as a training or temperament problem. It is worth ruling out medically in a dog whose behaviour has changed alongside weight gain, coat changes or reduced energy.

Hypothyroidism in Dogs: The Diagnosis Hiding Behind 'He's Just Got Lazy'

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There is a version of this dog in a great many Indian households.

Seven years old. Has put on weight even though nobody is feeding more. Not interested in walks the way he was. Coat has gone dull and thin. Seeks out the warmest spot in the house. Keeps getting ear infections. Everybody agrees he has just got lazy in middle age.

That is a diagnosis, it costs one blood test, and the treatment is a cheap daily tablet.

What the thyroid does, briefly

The thyroid gland sets metabolic rate. In hypothyroidism it produces too little hormone, and everything downstream slows, energy production, skin and coat turnover, temperature regulation, and to some degree cognition and mood.

Most canine cases result from progressive destruction of the gland itself. It is a middle-aged dog's disease, typically four to ten years, with medium and large breeds over-represented: Golden Retrievers, Dobermans, Boxers and Irish Setters among them. Uncommon in very small breeds, rare in puppies.

The signs, in the order owners notice them

Weight gain with no change in food. Usually the first thing, and usually attributed to age.

Lethargy. Sleeping more, less enthusiasm for walks, tiring sooner.

Coat and skin, the part that points at the thyroid specifically:

Symmetrical hair loss over the trunk and flanks, both sides equally
A "rat tail", hair thinning or lost along the tail. This one is close to a signature
Dull, dry, thinning coat generally
Darkened, thickened skin
Hair that does not regrow after clipping

Seeking warmth. Lying in sunny patches, under blankets, close to people. In Indian summer heat this can be missed entirely; it shows up in winter.

Recurrent ear and skin infections. Hypothyroidism suppresses immune function, so ear infections and skin infections keep coming back and get treated one at a time for years.

A slow heart rate, which your vet will notice.

Less commonly: facial changes giving a "tragic" expression, neurological signs, and reproductive problems.

The pattern to act on: any two or three of these together in a middle-aged dog. Individually each is easy to explain away. Together they are a blood test.

Diagnosis, and why one number is not enough

This deserves explaining, because it is where diagnosis goes wrong in both directions.

Diagnosis rests on clinical and historical findings suggestive of the disease, supported by laboratory findings. Not on a single value.

The complication is euthyroid sick syndrome: other illnesses, and some medications, suppress thyroid hormone readings in a dog whose thyroid is perfectly healthy. A dog that is unwell for an unrelated reason can show a low T4.

So a low total T4 alone should not start lifelong medication. The usual approach is total T4 alongside free T4 and TSH, interpreted together and in the context of the clinical picture.

Ask your vet directly: "Is this panel enough to be confident, or do we need free T4 and TSH?" It is a reasonable question and it prevents both a missed diagnosis and years of unnecessary tablets.

Treatment

Daily oral levothyroxine, usually twice daily, lifelong.

It is inexpensive, one of the cheapest chronic treatments in veterinary medicine.

What to expect, and the timeline matters because owners give up too early:

Improves	Roughly
Energy and demeanour	2-4 weeks
Weight, with appropriate feeding	1-3 months
Coat and skin	2-4 months, sometimes worse before better as old coat sheds

Monitoring. Blood levels are rechecked after starting to confirm the dose, then periodically for life. Requirements change, and a dose set three years ago is not necessarily right today.

Give the tablets consistently, same times, and consistently with or without food, since absorption varies.

The consequence of missing it

This is why it is worth pushing for the test.

A dog with untreated hypothyroidism spends years overweight, tired, itchy, and repeatedly treated for ear and skin infections that keep returning. Each infection is treated as its own event. The thyroid is never tested because the dog is "just getting older".

Meanwhile the weight worsens any arthritis present, the recurrent infections cost more than the test would have, and the dog's quality of life is meaningfully reduced for years.

It is one of the more frustrating things to find late, precisely because the fix is so straightforward.

The bottom line

Middle-aged dog. Gaining weight without eating more. Tired. Thinning coat on both flanks. Rat-like tail. Keeps getting ear infections. Seeks out warm places.

That is not age. That is a blood test, and if it comes back positive, a cheap daily tablet gives you a different dog inside two months.

Ask whether the panel includes free T4 and TSH, because a single low T4 in an otherwise unwell dog can be misleading in both directions.

Symmetrical, non-itchy hair loss on both flanks is the sign that most reliably separates this from a skin problem, see dog hair loss.