Dog & Cat HospitalCalabasas

Patient Education

Hypothyroidism

Hypothyroidism is the most common hormonal disorder in dogs — and once diagnosed, it's one of the easiest to treat, with most dogs showing dramatic improvement within weeks.

Hypothyroidism occurs when the thyroid glands produce too little thyroid hormone, causing the body's metabolism to slow down across every organ system. In dogs, the vast majority of cases result from the immune system gradually destroying thyroid tissue (lymphocytic thyroiditis) or from idiopathic atrophy — the gland simply wasting away for reasons that are not fully understood. Because thyroid hormone affects nearly everything, the signs are wide-ranging and easy to confuse with normal aging. The condition is rare in cats. At Dog & Cat Hospital Calabasas, hypothyroidism is diagnosed with a same-day blood panel and treated with straightforward daily medication — most dogs respond so well that owners describe their pet as "acting young again."

Signs & Symptoms

Hypothyroidism develops slowly, so signs often go unnoticed for months or years. The combination of weight gain and lethargy in a dog that isn't eating more is a common tip-off:

  • Weight gain without an increase in food intake
  • Lethargy, reduced activity, and reluctance to exercise
  • Coat changes — dry, brittle fur; excessive shedding; symmetrical hair loss on the trunk without itching
  • Skin thickening or a dull, rough texture
  • Slow heart rate (bradycardia)
  • Cold intolerance — seeking warm spots and shivering easily
  • Facial changes in some dogs — a "tragic" expression caused by thickening of the skin over the face (myxedema)
  • Reproductive irregularities in intact dogs
  • Rarely: peripheral neuropathy causing weakness, dragging of a limb, or vestibular signs

Because many of these signs overlap with normal aging and other conditions, bloodwork is essential for a definitive answer.

Causes & Risk Factors

Almost all canine hypothyroidism is primary — meaning the thyroid gland itself fails — rather than secondary to pituitary or brain disease. The two most common causes are:

  • Lymphocytic thyroiditis — the immune system attacks and progressively destroys thyroid tissue; may have a genetic component
  • Idiopathic follicular atrophy — the gland gradually shrinks and stops functioning without a clear trigger

Additional risk factors include:

  • Breed — Golden Retrievers, Doberman Pinschers, Irish Setters, Boxers, and many mid- to large-size breeds are over-represented
  • Middle age — most dogs are diagnosed between four and ten years old
  • Neutered status — spayed and neutered dogs may have a modestly higher risk in some studies
  • Prior treatment with certain drugs (sulfonamide antibiotics, for example) can transiently suppress thyroid function

Is your dog gaining weight and losing energy?

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How We Diagnose It

Diagnosis is based on bloodwork, but interpreting thyroid tests in dogs requires some nuance. Many factors — illness, stress, and certain medications — can lower thyroid hormone levels in an otherwise healthy dog (a phenomenon called the "euthyroid sick syndrome"). Our approach:

  • Total T4 (thyroxine) — a useful screening test; a normal value makes hypothyroidism very unlikely
  • Free T4 by equilibrium dialysis (fT4-ED) — more specific; particularly useful when T4 is low-normal or borderline
  • Thyroid-stimulating hormone (cTSH) — elevated TSH alongside low T4 is strong evidence for primary hypothyroidism
  • Complete chemistry panel and CBC — to look for concurrent issues, including the mild anemia and elevated cholesterol often seen with hypothyroidism

We review all results together with the clinical picture before making a diagnosis, and our in-house lab delivers results the same day.

Treatment Options

Treatment is straightforward: daily oral supplementation with synthetic thyroxine (levothyroxine). Key points for owners:

  • The medication is given once or twice daily — the specific schedule is tailored to each dog's response
  • Most dogs show visible improvement in energy within two to four weeks, with coat and skin changes taking three to four months to fully resolve
  • Dose adjustments are common early in treatment; recheck bloodwork at four to six weeks after starting, then every six months once stable
  • Medication must continue for life — hypothyroidism does not go away on its own, and stopping treatment causes signs to return
  • Generic and brand-name formulations are available; consistent use of the same formulation helps maintain steady thyroid levels

Hypothyroidism is one of the most gratifying diagnoses in veterinary medicine because the response to treatment is often dramatic and rapid.

When to See Our Team in Calabasas

If your dog has been gaining weight without eating more, seems sluggish, or has developed coat and skin changes, a thyroid blood panel is a sensible first step — especially in a middle-aged dog of a susceptible breed. The test is simple, the treatment is effective, and catching hypothyroidism early prevents unnecessary weight on the joints and other downstream effects. Our team at Dog & Cat Hospital Calabasas runs full thyroid panels in-house, with results available the same visit. Book an appointment and let us take a look.

Frequently Asked Questions

1What happens if a dog's hypothyroidism goes untreated?
Untreated hypothyroidism causes progressive weight gain, worsening lethargy, skin and coat deterioration, and, in some dogs, nerve problems affecting movement. While not immediately life-threatening, the slow metabolic decline significantly reduces quality of life and strains the heart, joints, and other organs over time.
2Can a dog with hypothyroidism live a normal life?
Absolutely — once the medication dose is properly calibrated, most hypothyroid dogs are indistinguishable from healthy dogs in energy, coat quality, and weight. The main commitment is remembering daily medication and periodic bloodwork.
3How long before I see improvement on thyroid medication?
Many dogs show improved energy and attitude within two to four weeks of starting levothyroxine. Coat and skin changes take longer — expect three to four months for significant improvement in hair regrowth.
4Is hypothyroidism common in cats?
No — naturally occurring hypothyroidism is very rare in cats. When a cat's thyroid levels are low, it is most often the result of treatment for hyperthyroidism (medication, surgery, or radioactive iodine) rather than a spontaneous disease.
5Can a dog recover from hypothyroidism without medication?
Spontaneous recovery does not occur. Hypothyroidism in dogs is caused by irreversible destruction or atrophy of thyroid tissue, so supplementation is required for life. Dogs that stop medication return to their previous symptoms.
6My vet said my dog's thyroid is borderline — do we treat or wait?
Borderline results require additional testing — particularly free T4 and TSH — and correlation with the dog's clinical signs. Some dogs with low-normal T4 are completely healthy (euthyroid sick syndrome); others genuinely have early hypothyroidism. We'll interpret the full picture rather than treating based on a single number.
7Can hypothyroidism cause aggression or behavioral changes in dogs?
Some dogs with hypothyroidism show behavioral changes, including irritability or sudden aggression, though this is less common than the physical signs. When behavioral changes accompany classic physical signs, ruling out hypothyroidism with bloodwork is worthwhile.
8How often does a hypothyroid dog need recheck appointments?
Most dogs need a recheck four to six weeks after starting or changing their dose, and then every six months once stable. Blood is drawn 4–6 hours after the morning dose to get an accurate peak measurement.

Is your dog gaining weight and losing energy?

A simple thyroid blood panel run in our on-site lab can confirm hypothyroidism the same day — and treatment often makes dogs feel dramatically better within weeks.

Book an Appointment