Hypothyroidism in Dogs: Weight Gain, Hair Loss, and Lethargy Explained

If your dog is gaining weight even though you haven't changed their food, losing hair in patches, or acting tired all the time, hypothyroidism should be on the list of possibilities. This is one of the most common hormonal disorders I diagnose in dogs here at Hearthstone Animal Clinic in Cypress, TX, and the good news is that once we identify it, treatment is straightforward and affordable.

The frustrating part for owners is that hypothyroidism creeps up slowly. The changes are subtle enough that many people assume their dog is just getting older. By the time they bring the dog in, the condition has often been developing for months.

What is hypothyroidism and why do dogs get it?

The thyroid gland sits in your dog's neck and produces hormones (primarily T4 and T3) that regulate metabolism. When the thyroid doesn't produce enough of these hormones, everything slows down: energy levels, skin cell turnover, body temperature regulation, and the rate at which the body burns calories.

In about 95% of cases, the cause is lymphocytic thyroiditis (an autoimmune condition where the immune system attacks the thyroid tissue) or idiopathic thyroid atrophy (the thyroid tissue simply shrinks and stops functioning). We rarely know why either happens. Unlike in humans, thyroid cancer is an uncommon cause of hypothyroidism in dogs.

Which dogs are most at risk?

Hypothyroidism typically shows up in middle-aged dogs, usually between 4 and 10 years old. Certain breeds have higher rates:

• Golden Retrievers
• Doberman Pinschers
• Irish Setters
• Dachshunds
• Cocker Spaniels
• Boxers
• Great Danes

That said, any breed can develop it, including mixed breeds. Spayed and neutered dogs have slightly higher rates than intact dogs, though the difference isn't dramatic enough to influence spay/neuter decisions.

What are the signs of hypothyroidism in dogs?

The classic triad is weight gain, hair loss, and lethargy, but not every dog shows all three. Here's what I typically see:

Metabolic changes

• Weight gain without increased appetite. This is the most common owner complaint. The dog isn't eating more, but keeps putting on pounds.
• Exercise intolerance. Walks that used to be easy now tire the dog out.
• Cold intolerance. Seeking out warm spots, reluctance to go outside in cooler weather.
• Mental dullness. The dog seems "off," less engaged, slower to respond.

Skin and coat changes

• Bilateral symmetric hair loss. Hair thins on both sides of the body, often along the flanks, tail ("rat tail"), and back of the thighs. The head and legs are usually spared.
• Dry, dull coat. Hair that doesn't regrow after clipping.
• Thickened skin. Some dogs develop a "tragic" facial expression from skin thickening (myxedema).
• Recurrent skin infections. Bacterial or yeast infections that keep coming back despite treatment.

If your dog has been treated repeatedly for allergies or skin issues without lasting improvement, thyroid testing should be part of the workup.

Less common signs

• Slow heart rate (bradycardia)
• High cholesterol on bloodwork
• Reproductive problems (in intact dogs)
• Neurological signs like weakness or nerve damage (rare)

How is hypothyroidism diagnosed?

Diagnosis requires bloodwork. The most common initial test is a Total T4 (thyroxine) level. If T4 is low, I typically run additional tests to confirm:

• Free T4 by equilibrium dialysis (fT4ED): More accurate than total T4, less affected by other illnesses or medications.
• TSH (thyroid-stimulating hormone): In true hypothyroidism, TSH is usually elevated because the pituitary gland is trying to stimulate a failing thyroid.
• Thyroglobulin autoantibodies (TgAA): Can detect autoimmune thyroiditis, sometimes before clinical signs appear.

The reason I don't diagnose based on T4 alone is that many things can temporarily lower T4 levels: other illnesses, certain medications (steroids, phenobarbital, sulfonamides), even stress. This is called euthyroid sick syndrome, where the dog has low thyroid numbers but doesn't actually have thyroid disease. A full comprehensive bloodwork panel helps rule out other conditions that might be causing similar symptoms or affecting test results.

Testing typically runs $150 to $300 depending on how many thyroid values we need to check.

What does treatment look like?

Treatment is levothyroxine (synthetic thyroid hormone), given orally once or twice daily for the rest of the dog's life. Brand names include Thyro-Tabs and Soloxine. Generic versions work well for most dogs.

This is one of the more affordable lifelong medications. Depending on your dog's size, expect to pay roughly $20 to $50 per month. Larger dogs need higher doses, so costs increase with body weight.

What to expect after starting treatment

Most owners notice energy levels improving within the first 2 to 4 weeks. That's usually the first sign that the diagnosis was correct and the dose is working.

Skin and coat changes take longer. You'll typically see improvement over 2 to 4 months. Hair regrowth can take up to 6 months in severe cases.

Weight loss requires the same discipline it always does: once metabolism normalizes, the dog still needs appropriate portions and exercise. The medication doesn't melt pounds off, but it does restore the body's ability to use calories normally.

Monitoring and dose adjustments

I recheck thyroid levels 4 to 6 weeks after starting medication, then every 6 months once we find the right dose. Blood should be drawn 4 to 6 hours after the morning pill for accurate "peak" levels. Dose adjustments are common in the first few months.

The American Animal Hospital Association guidelines recommend keeping post-pill T4 levels in the upper half of the normal range for good symptom control.

Can hypothyroidism be confused with other conditions?

Yes, and this is where diagnosis gets tricky. Several conditions mimic hypothyroidism or can occur alongside it:

• Cushing's disease (hyperadrenocorticism): Also causes hair loss, weight gain, and lethargy. Sometimes dogs have both conditions.
• Allergies and other skin diseases: Hair loss and skin infections overlap significantly.
• Age-related slowing: The most common reason hypothyroidism goes undiagnosed for months. "He's just getting old" is often what owners tell themselves.

This is why I don't recommend starting thyroid medication based on symptoms alone or a single low T4 value. The diagnosis should be solid before committing to lifelong treatment.

Are there side effects from thyroid medication?

Levothyroxine is generally very safe. Side effects are uncommon but can include:

• Increased thirst and urination (usually means the dose is too high)
• Restlessness or hyperactivity (dose too high)
• Rapid heart rate or panting (dose too high)

If you notice these signs, let your veterinarian know. A dose reduction usually resolves them quickly. Overdose is rare with oral medication at prescribed doses.

When should you have your dog tested?

If your dog is middle-aged or older and showing any combination of unexplained weight gain, hair loss (especially on the body but not the head), chronic skin problems, or a noticeable drop in energy, thyroid testing is reasonable. It's a simple blood draw and can save months of guessing.

For dogs in high-risk breeds, I sometimes include thyroid screening as part of routine wellness bloodwork starting around age 5, even if symptoms aren't obvious yet.

The bottom line for Cypress dog owners

Hypothyroidism is common, underdiagnosed, and very treatable. If this was my dog showing these signs, I'd want the bloodwork done sooner rather than later. The medication works, it's affordable, and most dogs return to their normal selves within a few months of starting treatment.

If your dog has been slowing down, gaining weight, or losing hair and you're not sure why, schedule a visit at Hearthstone Animal Clinic so we can figure out what's going on. You can reach us at (281) 859-9244. We see these cases regularly here in Cypress, TX, and I'd rather catch it early than watch a treatable condition drag on.