Hypothyroidism in dogs: symptoms and treatment

Overview

The thyroid has 2 lobes at the front of the neck beside the trachea. It produces thyroxine, or T4, and triiodothyronine, or T3. These hormones maintain metabolism. Their deficiency slows cell activity. Effects therefore appear in several organ systems, rather than only the coat or weight.

Hypothyroidism is one of the common hormonal diseases in dogs. Estimated overall prevalence is about 0.2–0.4% of all dogs. It is diagnosed considerably more often in some breeds. Signs often develop slowly over months or years. However, weight gain or a deteriorating coat is not direct proof of hormone deficiency.

Causes

More than 95% of clinical cases result from damage to the thyroid gland itself. This is called primary hypothyroidism. Common underlying causes are lymphocytic inflammation and gland atrophy. In immune-mediated inflammation, the body damages its own thyroid tissue.

With atrophy, fat and connective tissue replace normal gland tissue. This may be the end stage of inflammation. Genetic susceptibility contributes to some cases. Environmental factors may play a part in triggering disease. An individual dog's exact trigger generally cannot be inferred from signs.

Less common causes include a thyroid tumour, pituitary disease or a congenital developmental disorder. TSH from the pituitary directs thyroid function. Pituitary damage can reduce this stimulation. Another illness can also lower thyroid values without true hypothyroidism.

Risk factors

The disease is most often diagnosed in dogs aged 4–10 years. An average age at diagnosis of 6.8 years has been reported. Medium and large breeds are often represented. No clear overall sex susceptibility has been demonstrated. Rarely, younger dogs can have a congenital form.

Susceptible breeds include Golden Retrievers, Dobermanns, Irish Setters and Airedale Terriers. Breed also affects interpretation of normal hormone results. Greyhounds, for example, often have lower T4 and free T4 than other breeds. Sled dogs in hard training can also have low levels without disease.

Basenjis, Salukis and Whippets can also have T4 below general reference ranges without thyroid disease.

Typical symptoms

The most common pattern relates to a slowing metabolism. Changes you may notice at home include:

• Tiredness, sleepiness and reduced activity.
• Weight gain without increased appetite.
• Poor exercise tolerance and reluctance to move.
• Feeling cold or seeking warm places.
• A dry, brittle or dull coat.
• Thinning hair, particularly on the trunk or tail.
• Slow regrowth of clipped hair.

Hair loss is often symmetrical and does not itself itch. Skin may darken. Recurring skin and ear infections may form part of the picture. Infected skin may nevertheless be itchy.

Less common signs can include constipation, vomiting or diarrhoea. Some dogs have a slow heart rate or abnormal rhythm.

Complications and consequences

In severe hypothyroidism, substances can accumulate in skin, making it thickened and swollen. This is called myxoedema. It often appears as puffiness of the forehead and face. Neurological changes such as facial paralysis or balance problems occur less commonly. They do not always resolve with hormone treatment.

Entire dogs may also have reproductive problems. Females may have absent or irregular seasons. Males may have reduced fertility or sex drive. Congenital hormone deficiency can disrupt skeletal and nervous system development. A correctly diagnosed and treated adult dog usually has a good outlook.

Fat deposits may form in the cornea. Facial swelling can be associated with slight upper eyelid drooping. In congenital disease, a puppy may remain disproportionately small. Early hormone deficiency can also affect mental development.

Diagnosis at the vet

The vet examines your dog and assesses diet, weight changes and medicines alongside signs. Diagnosis combines signs and laboratory investigations. Basic tests may show high cholesterol or triglycerides. Mild anaemia is also possible. These findings alone are not disease-specific.

Total T4 is a common initial screening test. Free T4 means the hormone fraction not bound to proteins. TSH adds to assessment of thyroid function. In confirmed hypothyroidism, TSH can be normal in 20–40% of dogs. Normal TSH therefore does not rule out disease.

Low T4 alone is insufficient for diagnosis. Other illnesses can lower it too. Severe general illness can also lower free T4. Where possible, thyroid tests are therefore performed after another illness has settled. Ageing can also reduce T4.

Glucocorticoids, phenobarbital and sulfonamides, for example, can alter results. Do not stop medicines yourself for testing. Antibodies against thyroid hormones can sometimes falsely raise measured hormone results. Free T4 measured by equilibrium dialysis may then help interpretation.

Thyroglobulin antibodies, or TgAA, may indicate autoimmune thyroiditis. However, this test does not measure whether thyroid hormone production is adequate. It is used alongside other tests when needed. If signs and basic results do not fit, the vet may investigate other illnesses too.

Cholesterol is raised in about 80% of dogs with hypothyroidism. TSH can, in turn, be raised in about 8–10% of dogs with another general illness. This also highlights joint interpretation of findings. T3 can be low, normal or sometimes high in hypothyroidism. It is therefore not usually a primary diagnostic measure.

Thyroid ultrasound by an experienced examiner can add to investigation of an unclear case. It assesses gland size and tissue echogenicity, among other features. Thyroid scintigraphy is another specialist option. A TSH stimulation test measures remaining thyroid functional reserve. Availability of the required TSH limits its use.

Total T4 screening has about 90% sensitivity. TSH stimulation samples are taken before stimulation and after 6 hours. Sample handling also matters: warming can falsely raise free T4.

Treatment

The vet prescribes treatment and medicines. Confirmed hypothyroidism is treated with the synthetic thyroid hormone levothyroxine. Medication is usually lifelong. It restores circulating hormone to adequate levels. Low thyroid values caused only by another illness generally do not require replacement.

Energy and mental alertness should begin improving in about 2 weeks. Recovery of coat, skin and weight can take up to 3 months. Slow hair regrowth therefore does not immediately mean treatment has failed. If signs persist despite appropriate hormone levels, the diagnosis is reviewed.

The first treatment recheck is often about 4 weeks after medication starts. A range of 4–8 weeks is also used. Blood sampling time relative to medication is important: a sample assessing peak concentration is usually taken 4–6 hours afterwards. Follow your own practice's sampling instructions.

Once control is established, T4 checks are usually performed 1–2 times a year. This often means an interval of about 6–12 months. Improvement in your dog's condition is the main sign of success. The amount of medicine is adjusted according to individual signs and blood results.

Food can affect levothyroxine absorption. Keep medication timing relative to meals consistent with instructions, including on testing days. Excessive hormone activity may cause restlessness and panting. Weight loss, increased thirst or a fast heart rate may also suggest an excessive effect.

If your dog has antibodies against thyroid hormones, free T4 may also be needed for monitoring. Target results are interpreted in the context of hormone replacement. An incorrect original diagnosis is the most common reason for treatment failure. The vet then plans reassessment, rather than you trying more medicine yourself.

Excessive hormone activity can also increase urination.

Prevention

Autoimmune hypothyroidism involves genetic susceptibility. There is no reliable known home prevention. Breeding choices can consider the inherited component. Some breed clubs recommend not breeding from a dog with hypothyroidism. These recommendations call for a healthy mate with no close relatives affected by the same disease when serious inherited disease occurs in the dog's close family.

Breeding assessments may use thyroglobulin antibody testing to help identify autoimmune inflammation. However, antibodies do not replace assessment of hormone function and signs. In everyday life, the main aim is to recognise persistent changes and obtain the right diagnosis. Do not treat weight gain alone with hormone medicine yourself.

Important for dog owners

Keep a weight diary and photograph the coat in the same lighting before treatment and at rechecks. Record alertness, movement, feeling cold and skin signs. Bring every medicine and supplement with details of use. Note the medicine time on testing days. This helps interpretation because peak T4 is usually assessed 4–6 hours after medication.

Do not increase, reduce or stop hormone treatment based only on weight or coat. Report new restlessness, thirst or weight loss to the vet treating your dog. Check the rules for your activity before entering an event. Do not withhold medication in order to compete.

Frequently asked questions

Does low T4 confirm hypothyroidism?

No. Low T4 may reflect another illness, medication or a breed-typical concentration. Diagnosis requires assessment of signs and further tests. Review results carefully with your vet alongside other findings before starting lifelong replacement.

Can TSH be normal in an affected dog?

Yes. In confirmed hypothyroidism, TSH is within the reference range in about 20–40% of dogs. A normal value therefore does not exclude disease. TSH is always interpreted alongside signs and total or free T4.

How quickly does medication help?

Energy should improve in about 2 weeks. Skin, coat and weight recovery may take up to 3 months. Treatment is not judged solely by rapid hair growth. Agreed blood tests and monitoring help establish the right balance.

Will my dog need medicine for life?

Hormone replacement for confirmed hypothyroidism usually continues throughout life. Medication replaces missing hormone production. Once stable, rechecks are generally 1–2 times a year. Your dog's vet plans individual monitoring and any extra checks.

Is itchy skin a typical sign?

Hair thinning from hypothyroidism does not usually itch by itself. However, secondary bacterial skin infection can cause itching. Recurring ear infections may also occur. An itchy dog's skin therefore needs separate assessment rather than automatically attributing the sign solely to thyroid disease.