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What is canine hypothyroidism?
The thyroid gland, in the neck, makes thyroid hormones (T4 and T3) that set the speed of the metabolism, a bit like a thermostat. The brain's master gland (the pituitary) sends a signal, TSH, telling the thyroid to produce them. In hypothyroidism, the thyroid wears out and can't keep up: hormone levels fall, everything slows, and the pituitary pushes out more and more TSH trying to compensate. It is a common disease, but also one of the most over-diagnosed, which is why it is confirmed carefully, never on a single number.
Lymphocytic thyroiditis
The dog's immune system mistakenly attacks its own thyroid and slowly destroys it. It is partly a genetic predisposition, not a result of diet.
Gland atrophy
Thyroid tissue shrinks and is replaced by fat. It is thought to often be the burnt-out end-stage of an earlier thyroiditis: the two are really one process moving forward.
Rare forms
More than 95% of cases come from the gland itself. Rarely, the brain's signal is at fault (central form), or a puppy is born with a failing thyroid: abnormal growth and slow development, worth having checked.
In cats it is the opposite: the common thyroid disease is hyperthyroidism (too much hormone), with mirror-image signs (weight loss, hyperactivity). Two opposite diseases, two different approaches.
What too little hormone does to the body
Thyroid hormones are the metabolism's accelerator. When they run low, every organ idles. Signs settle in slowly, over months, and are easily mistaken for 'he's just getting older.'
Metabolism and weight
The body burns less: the dog gains weight without eating more, and tires quickly. It is the most deceptive sign, because it gets blamed on aging.
Energy and mood
Lethargy, less enthusiasm, a sense that the dog 'aged overnight.' Some seek out warmth and shiver easily (cold intolerance).
Skin and coat
Dull, dry coat, hair loss on both sides of the body (symmetrical, non-itchy), 'rat tail,' darker skin, recurrent ear and skin infections.
Heart
The heart rate may slow slightly. These changes are subtle, picked up by the veterinarian, and reverse with treatment.
Recognizing the signs
There is no urgency as it sets in: these signs appear slowly and call for an appointment, not the emergency room. The useful reflex: a middle-aged dog that is slowing down, gaining weight without eating more, and losing coat deserves a thyroid workup, rather than being written off as age.
General signs
- Lethargy, less play and walking
- Weight gain without eating more
- Heat-seeking, cold sensitivity
- A 'dimmed' look, falling asleep easily
Skin and coat
- Dull, dry coat that regrows poorly after a clip
- Symmetrical hair loss, 'rat tail'
- Darker, thickened skin
- Recurrent ear and skin infections
Advanced forms (rare)
- Puffy face with a 'sad' expression (myxedema)
- Very thickened skin, folds on the forehead
- Weakness, a very subdued dog
- Signs that will not ease: see the vet to reassess
My dog is overweight: is it his thyroid? It is the most common question, and the answer surprises people: the vast majority of overweight dogs are NOT hypothyroid, they simply eat too much or move too little. Hypothyroidism does cause weight gain, but without an increase in appetite and alongside the other signs (lethargy, coat, cold sensitivity). Being overweight alone justifies neither a test nor thyroid treatment: it is the whole picture that counts, and your veterinarian sorts it out.
When is it urgent?
Hypothyroidism is a slow disease: it almost always calls for an appointment, not the emergency room. There is one true emergency, rare, worth knowing: myxedema coma, the endpoint of severe, long-untreated disease. Don't wait for a literal coma: these signs together, in a dog, mean go now.
- Dog cold to the touch, low body temperature
- Very weak, collapsed or 'floppy,' won't get up
- Not responding normally, dazed or unconscious
- Sometimes a puffy, swollen face
How is the diagnosis made?
This is where everything is decided, because hypothyroidism is one of the most over-diagnosed diseases in dogs. The key message: a single low T4 is NEVER enough to conclude, and a dog is not put on a lifelong pill based on one isolated number. The diagnosis is built from the signs plus a small panel of tests, interpreted together.
Why a low T4 is not enough: Why a low T4 isn't enough: many things push it down with no thyroid disease at all. An unrelated illness (a sick dog often has a falsely low T4) or certain medications (for example cortisone, some anti-seizure drugs, some antibiotics) lower the result. That is why we don't test a sick or recently-medicated dog, and why you should tell the vet EVERY medication and supplement your dog gets.
The signs, first
Testing only comes after a good reason to suspect the disease: lethargy, weight gain, skin and coat changes, cold sensitivity, in a middle-aged dog. Without suggestive signs, we don't 'screen' the thyroid at random.
Total T4: a screening test
A normal T4 makes hypothyroidism very unlikely and is reassuring. A low T4 is only a yellow light: it says look further, not conclude.
The confirmation panel
To confirm, we add free T4 by dialysis (the most reliable measure) and canine TSH. Note: a normal TSH does not rule the disease out (it is normal in some truly affected dogs). Antibodies that flag an autoimmune thyroiditis can also be measured.
Ruling out false leads
We make sure the dog isn't sick with something else and that no medication is skewing the result. If needed, we remove the interference and retest later: better to wait than to treat wrongly.
Supporting clues
Bloodwork often shows clues the veterinarian notices: high cholesterol, a mild anemia. They support the diagnosis without proving it on their own; it is the whole picture that settles it.
The treatment: one, for life
Unlike the hyperthyroid cat, who has a choice among several treatments, canine hypothyroidism has one simple treatment: replace the missing hormone. There is no surgery, no radioactive iodine, no special diet. Since the gland is worn out, we don't 'cure' it: we make up for it, every day.
Levothyroxine (synthetic T4)
A daily pill of synthetic thyroid hormone, identical to the one the thyroid no longer makes. It is not a 'strong' medication: it is a replacement, taken for life. The starting dose is set by your veterinarian based on weight, then adjusted: it belongs to the two of you, there is no dose to 'guess' at home.
How to give it
Consistently, ideally on an empty stomach, and above all ALWAYS the same way: food reduces absorption, and inconsistency unsettles both the dog and the follow-up bloodwork. Often once a day, sometimes twice: your veterinarian tailors it to your dog.
Monitoring
We recheck a blood test (a 'post-pill' T4, drawn a few hours after the dose) a few weeks after starting or any dose change, then adjust until stable, and after that once or twice a year. The number is always read alongside how the dog is doing.
What to expect
Energy and brightness come back quickly, often within one to two weeks: it is the first 'waking up' owners notice. Weight and especially the coat take longer, up to about three months, and the coat may even look worse before it regrows: that is not a treatment failure.
Safety: must-remember
- NEVER adjust or stop the dose yourself: stopping lets the disease quietly return.
- Missed dose: give it as soon as you remember, unless the next one is close; never double up.
- Too much hormone (over-dosing) gives the opposite of the disease: hyperactivity, panting, more thirst and urination, weight loss despite appetite, fast heart rate. It is not an emergency, but call promptly to reassess the dose; don't correct it alone.
- If the dog swallows the bottle (a large amount at once): that is a poisoning, call your veterinarian or a poison line right away, don't treat it like a simple missed dose.
What to expect long-term?
It is one of the most rewarding chronic diseases to treat: with a daily pill and regular monitoring, the prognosis is excellent and the vast majority of dogs return to a completely normal life, with a normal life expectancy.
For the treatment to succeed
Consistency
Same time, same way, every day: consistent dosing is what keeps the level steady and the monitoring meaningful.
The follow-up
The recheck blood tests are how the vet finds the right dose. Keeping them is not a formality: it is the treatment.
Patience
Energy returns in weeks, the coat in months, and it may look worse first. Trust the process, don't stop early.
Living with it day to day
Small comfort touches
Daily actions
- Give levothyroxine consistently, the same way every day
- Monitor weight and note any changes
- Watch for energy to return first, then the coat improving over a few months
- Keep the follow-up blood tests: that is what sets the right dose
- Report any sign of over-dosing (hyperactivity, panting, thirst, weight loss)
- Tell the vet about any other medication or supplement given
Avoid these
- Adjusting or stopping the dose yourself, even if the dog seems fine
- Doubling up a missed dose
- Concluding it has failed because the coat looks worse in the first weeks
- Blaming noisy breathing or regurgitation 'on the thyroid' without an assessment
Flag for the veterinarian
- Some herding breeds carry a known genetic drug sensitivity: mention the breed
- A puppy growing poorly or developing slowly: have it seen
- Any new breathing, neurological, or digestive sign
Frequently asked questions
Can it be cured?
Can I stop the pill when he's doing better?
My dog is overweight: is he hypothyroid?
Will treating the thyroid fix his noisy breathing or changed bark?
Which breeds are most at risk?
Can a puppy be hypothyroid?
How long before I see a change?
Is it my fault? Did I feed him wrong?
See also
This guide is provided for informational and educational purposes only. It does not constitute veterinary medical advice and is not a substitute for a consultation with a qualified veterinarian. Every animal is unique; their health must be evaluated individually. If you have concerns about your dog's health, contact our clinic or consult a veterinarian promptly.
Is your dog slowing down and gaining weight?
A well-run thyroid workup, with the right tests, makes all the difference. Our team is here to assess your dog and, if needed, start the right treatment.