Hypothyroidism In Dogs

Written by Shula Berg BVSc CertAVP(GSAS) GPAdvCert(SASTS) MRCVS
Clinically reviewed by Elizabeth McLennan-Green BVM&S CertAVP(SAM) MRCVS

Table Of Contents

 - Overview
 - Symptoms
 - Diagnosis
 - Treatment
 - Outlook

What is Hypothyroidism?

Hypothyroidism, or an underactive thyroid, is a common and very manageable condition in dogs. It occurs when the paired thyroid glands in the neck are not producing enough thyroid hormone, though 75% of the glands have to stop functioning before symptoms are seen. 

In roughly half of cases, the thyroid gland stops producing hormones due to inflammation known as lymphocytic thyroiditis. This is an autoimmune condition, occurring when the body attacks its own cells, and is more prevalent in certain breeds of dog. In the other half of cases, the thyroid gland degenerates with age. Clinically, there is little difference in the two mechanisms of disease, and both types are treated the same way. 

Hypothyroidism can affect any breed or sex of dog, but is most common in middle aged or older dogs. Hypothyroidism due to lymphocytic thyroiditis is more common in purebred dogs, especially Golden Retrievers, Great Danes, Beagles, Cocker Spaniels, Boxers and Shetland Sheepdogs. 

What are the symptoms of Hypothyroidism?

Thyroid hormones affect many body systems, so dogs with hypothyroidism can show a wide variety of symptoms. Most of these are non-specific, meaning they can be caused by other conditions as well. 

The thyroid gland can slowly reduce output over months, or even years, so many symptoms develop gradually. Dogs with hypothyroidism often experience lethargy, excessive weight gain and exercise intolerance, though the extent of these changes may not be fully realised until they improve after starting treatment. 

Hypothyroidism can cause a range of dermatological problems, including poor quality, dry and/or thin hair coat, skin infections, greasy skin, and pigment changes.  Dogs may experience a loss of hair, especially over the flanks and tail (often described as having a “rat tail”). 

Many other body systems can be affected, with symptoms including neurological disease, heart disease, reduced tear production, and small intestinal bacterial overgrowth. 

Which tests are used to diagnose Hypothyroidism? 

Since symptoms are not specific, it is important to confirm a diagnosis before treating. Symptoms of hypothyroidism could be due to another condition, so other tests may be required to obtain a reliable diagnosis. Initially, a general blood screen including thyroid hormone levels (T4) may be performed. This helps to screen for other conditions, as well as assessing overall thyroid hormone levels. It is important to note that thyroid levels can be artificially lowered by other disease, so the next step is to test levels of thyroid stimulating hormone (TSH). If the thyroid is truly underactive, T4 levels will be low whilst TSH levels will be high. 

Uncommonly, T4 and TSH levels do not confirm hypothyroidism in a patient where no other cause for symptoms can be found. In these cases, it may be necessary to repeat the tests 4-6 weeks later. Rarely, we have to do additional tests, such as measuring free-T4 (the metabolically active fraction of the hormone) or thyroid autoantibodies. Some medications can affect thyroid hormone levels, so occasionally these may need to be stopped for a short period before thyroid tests are performed. 

How is Hypothyroidism treated?

Hypothyroidism is not curable, but can be managed with medication (thyroid hormone replacement therapy). Levothyroxine is administered once or twice daily, given as tablets or liquid. Absorption is reduced by food, so although tablets can be given in food, it is important to keep the timings of medication and feeding consistent each day. 

Blood tests are required to monitor response and guide dose adjustments, as not every dog will require the same dose. Although rare, overdose of thyroid hormone can be dangerous, so medication is always started at a low dose and gradually increased. Bloods are usually checked around two weeks after starting treatment, or after the dose is changed. Once the patient is stable, blood tests can be performed less frequently, often every 6-12 months ongoing. It is important to give the medication as normal on the day of the blood test, and it is usually best for the sample to be taken around 3 hours after dosing. Your vet will be able to advise on the timings. 

What is the outlook for dogs with Hypothyroidism? 

Hypothyroidism has an excellent prognosis, however, treatment is lifelong and requires regular monitoring. Coat changes can take 3-6 months to resolve; often damaged hair is shed before better quality hair grows in. This may look like a worsening of hair loss at first, but is a positive sign. Symptoms such as lethargy and mental dullness can start improving within a few days of starting treatment, and many dogs will lose up to 10% of bodyweight in the first three months.  

Disclaimer

Please note that the content made available on this webpage is for general information purposes only. Whilst we try to ensure that at the time of writing all material is up to date and reflects industry standards, we make no representation, warranties or guarantees that the information made available is up to date, accurate or complete. Any reliance placed by yourselves is done so at your own risk.

Page last reviewed: 25^th March 2024

Next review due: 25^th March 2026