Why is my dog gaining weight and losing hair? — Symptoms and treatment of canine hypothyroidism

WeightQ&AMeongsiljang Veterinary Advisory Board

We’ve put together everything pet owners need to know about canine hypothyroidism, from its definition and causes to key symptoms, diagnostic methods, and lifelong management.

What is hypothyroidism in dogs?

A middle-aged Labrador Retriever lying in the living room, looking lethargic

Hypothyroidism in dogs is an endocrine disorder in which insufficient thyroid hormone production slows down the body’s metabolism. Early detection is truly key. If you notice gradual weight gain, reluctance to go for walks, or thinning fur, these may not simply be signs of aging but could indicate a hormonal issue. However, because these symptoms often develop slowly and insidiously, they are easy to overlook. Diagnosis is made by evaluating clinical signs alongside blood test results, and with consistent treatment using synthetic thyroid hormone medication, symptoms can improve significantly.

What causes it?

Most cases of hypothyroidism in dogs are "primary," meaning the problem lies within the thyroid gland itself in the neck. There are two main causes. The first is lymphocytic thyroiditis, an autoimmune condition where immune cells gradually attack and damage the thyroid gland. The second is idiopathic thyroid atrophy, where the thyroid tissue progressively shrinks and loses function. This condition most commonly affects purebred medium- to large-sized dogs between the ages of 3 and 6.

Suspect hypothyroidism if you notice these symptoms

Thyroid hormones act like the body’s “speed regulator.” When levels are low, the entire body slows down. Check for these common signs:

Weight gain: Your pet gains weight despite eating the same amount of food.

Lethargy: They refuse walks or tire quickly.

Symmetrical hair loss: The coat thins on the sides and base of the tail (without itching).

Skin discoloration: The skin darkens and dandruff increases.

Cold sensitivity: They constantly seek out warm spots.

Bradycardia: Their heart rate slows down.

An illustration of a puppy showing symmetrical hair loss on its sides and tail area.

Distinguishing simple aging from hypothyroidism

Item	Simple aging	Hypothyroidism
Weight change	Muscle decreases and weight gradually declines or is maintained	Food amount stays the same but weight gradually increases
Coat condition	Thins overall	Falls out symmetrically on both sides and does not regrow well after clipping
Activity level	Gradually decreases with age	Generally becomes lethargic and tires easily
Skin	Dry	Dark pigmentation · dandruff
Cold reaction	Normal	Unusually sensitive to cold

Hypothyroidism appears insidiously and gradually, so it is easily mistaken for aging. If several items apply at the same time, it is best to get a thyroid hormone test at the hospital

How is it diagnosed?

At the veterinary clinic, blood tests are used to check total T4, free T4, and thyroid-stimulating hormone (TSH) levels together. According to veterinary textbooks, a definitive diagnosis of hypothyroidism cannot be made based on blood test results alone. While low T4 and high TSH levels are important clues, a final diagnosis requires a comprehensive evaluation of clinical symptoms, test results, and the long-term response to levothyroxine treatment. Blood test results can be distorted by other systemic diseases or steroid use, so we recommend retesting once your pet has recovered if they are not feeling well. A single blood test typically costs between $40 to $70.

A veterinarian performing a blood test on a dog’s front leg at the clinic.

These breeds are especially prone

Medium- and large-breed purebred dogs such as Golden Retrievers, Doberman Pinschers, Irish Setters, Boxers, Great Danes, and Dachshunds are known to be more prone to hypothyroidism. The condition typically begins in middle age, usually between 3 and 6 years old, so we recommend including thyroid hormone testing during routine health checkups starting from this age. If hypothyroidism is left untreated for a long time or becomes severe, it can progress to a medical emergency known as "hypothyroid crisis," characterized by sudden lethargy, hypothermia, and bradycardia. Although not all cases present with coma, if symptoms worsen rapidly, immediate veterinary care is essential.

Treatment is one pill for life

Treatment is straightforward: your pet takes a synthetic thyroid hormone called levothyroxine every day. At the start, it’s usually given twice daily (20 mcg/kg, every 12 hours), and once clinical signs have improved sufficiently, many pets can be transitioned to once-daily dosing. Your veterinarian will determine the exact dose and schedule based on your pet’s weight. Within a few days of starting treatment, lethargy typically begins to improve, though it may take several months for the coat to return to normal. Four to eight weeks after starting medication, blood tests are used to recheck hormone levels and adjust the dose if needed. Although lifelong treatment is required, with proper management, your pet can enjoy a normal lifespan.

A puppy, now lively again after taking its medicine, playing in the yard.

Manage it at home like this

Administer at the same time each day: Consistency is key. Fluctuations in dosing intervals can cause unstable blood levels.

Maintain consistency with food intake: Whether given on an empty stomach or after meals, stick to one method, as food affects absorption.

Regular monitoring: Even after stabilizing the dosage, schedule blood tests every six months.

Weight management: Appetite often returns during early treatment, increasing the risk of overeating. Consult your veterinarian to adjust portion sizes accordingly.

Caution with calcium and iron supplements: These can interfere with drug absorption, so space out their administration times.

Important things to know once treatment begins

Levothyroxine can cause symptoms resembling hyperthyroidism—such as restlessness, increased thirst, frequent urination, and weight loss—if overdosed. However, because dogs metabolize and excrete thyroid hormones quickly, these side effects are rare. If you notice any of these signs during the initial dose-adjustment period, do not stop the medication on your own; contact your veterinarian to determine whether a dosage reduction is needed. Conversely, never increase the dose simply because symptoms persist.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently asked questions

Once diagnosed, does my pet need to take medication for the rest of its life?

Yes, since damaged thyroid tissue cannot regenerate, lifelong medication is the standard approach. However, the medication itself is safe and well-tolerated, so with proper management, your pet can live a normal, healthy lifespan.

What happens if I miss a day of medication?

Missing a dose for a day or two isn’t usually a major concern, but please administer the medication as soon as you remember. Never give two doses at once to make up for a missed one. If missed doses become a recurring issue, it may cause fluctuations in the results of the next blood test.

Can it be replaced with food or supplements?

No. Hypothyroidism is a condition caused by a deficiency of the hormone itself, so it cannot be resolved with diet or natural supplements. The only proven treatment is administering levothyroxine.

Will the hair grow back after treatment?

Yes, in most cases, new hair will grow back within 2 to 4 months. Skin discoloration will also gradually fade. However, weight loss requires both dietary management and exercise.

Can puppies get it too?

Although rare, congenital hypothyroidism can occur. If you notice signs such as slow growth, a stunted body size, or delayed cognitive development, early testing is essential.

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References

[1] Ettinger SJ, Feldman EC. Textbook of Veterinary Internal Medicine, 8th Edition, Chapter Hypothyroidism

[2] Scott-Moncrieff JC. Canine Hypothyroidism. Vet Clin North Am Small Anim Pract. 2007;37(4):709-722

[3] Mooney CT, Peterson ME. BSAVA Manual of Canine and Feline Endocrinology, 4th Edition