Hypothyroidism in Dogs: Signs, Testing and Treatment

By Tailwisp Editorial TeamUpdated October 17, 20263 min read

Weight gain despite eating little, a dull coat and constant sleepiness can all trace back to a sluggish thyroid.

What it is

The thyroid gland controls metabolism. In dogs, hypothyroidism is most often caused by immune-mediated destruction or shrinkage of the gland. Metabolism slows, which affects almost every organ.

Who is affected

It is usually seen in middle-aged dogs, between about four and ten years, and more often in medium to large breeds such as golden retrievers, Labradors, Dobermans, boxers, Irish setters and cocker spaniels. Neutered dogs are more commonly affected.

Signs

• Weight gain without more food. See dog obesity risks
• Lethargy and reduced exercise
• Cold intolerance
• Dry, dull coat and hair loss, often symmetrical on the trunk and tail. See dog hair loss causes
• Recurrent skin and ear infections. See pyoderma in dogs and dog ear infections
• Darkened skin
• Slow heart rate
• Occasionally nerve problems, such as facial droop or vestibular signs. See vestibular disease in dogs

Diagnosis

Your vet will assess signs and bloodwork. A total T4 test is often a screening, followed by free T4 by equilibrium dialysis, TSH and sometimes antibodies. Because illness and some drugs can lower T4, interpretation needs care. Testing too early or during illness can mislead.

Treatment

Synthetic thyroid hormone, usually levothyroxine, is given once or twice daily. Dose is based on weight and adjusted by follow-up blood tests, often after four to eight weeks and then periodically. Many dogs show more energy within a couple of weeks, and skin and coat improve over months.

Giving the medication

1. Give at consistent times each day, with or without food as your vet advises.
2. Keep the dosing consistent, since feeding patterns can change absorption.
3. Do not skip doses.
4. Store tablets away from heat and moisture.
5. Bring a list of all supplements to your vet.

Monitoring

Bloodwork recheck times differ between dogs. Overdosing can cause restlessness, weight loss and a fast heart rate, so report such signs.

Living with hypothyroidism

Keep your dog active, feed measured portions and manage skin and ear problems promptly. With treatment, most dogs return to normal life expectancy.

Common mistakes

• Assuming weight gain is just overeating
• Starting treatment without proper testing
• Missing recheck tests
• Stopping medication when your dog improves

Troubleshooting

• No improvement after weeks: ask about retesting and other causes.
• My dog is hyper on medication: call your vet.
• Skin problems keep coming back: discuss further workup.

A simple plan

1. Get the right tests.
2. Start prescribed therapy.
3. Recheck on schedule.
4. Maintain weight and activity.

Real-life examples

A golden retriever regains energy and loses weight on treatment. A Doberman's recurrent ear infections finally clear. A cocker spaniel's coat regrows over several months. Treatment works for most dogs.

Staying consistent

A small daily pill makes a large difference once the diagnosis is confirmed.

Questions to ask your vet

Ask which tests were used, what T4 range is the target, how soon to retest after starting treatment and what changes you should expect in energy, weight and coat. Ask whether your dog's other medicines or supplements could affect the thyroid results or the absorption of levothyroxine.

Weight and exercise plan

Hypothyroid dogs usually need fewer calories until treatment takes effect. Measure meals, track weight every two weeks and add walks gradually as energy returns. See how to help an overweight dog lose weight.

Skin and ear care

Dogs with low thyroid hormone often develop recurring skin and ear infections that clear only when the thyroid is treated as well. Keep ears clean and dry, treat infections promptly and ask your vet about shampoo and ear care routines. See how to clean a dog's ears.

The takeaway

Hypothyroidism is common and very treatable. Proper testing and monitoring make treatment safe and effective.

This article is general educational information, not veterinary advice. Every dog is different. If your dog is unwell, or you are unsure about a symptom, diet change or medication, contact your veterinarian.