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Hypothyroidism in dogs: T4, TSH and the blood work clues

An underactive thyroid slows the whole body down: weight gain, tiredness, coat problems. It is common in dogs, but also over-diagnosed, because many other illnesses and medicines lower the thyroid hormone on a blood test. Here is how vets read the thyroid results.

Written by the AI DiagAnimals editorial teamReviewed by the AI DiagAnimals TeamUpdated Our editorial policy

The essentials

Hypothyroidism is a lack of thyroid hormone, usually from immune damage to the thyroid, diagnosed on average at about 7 years of age. Routine blood work often shows a high fasting cholesterol and a mild anaemia. A low total T4 alone is not enough: other illnesses and drugs lower it. The diagnosis rests on a low T4 and free T4 with a high TSH, interpreted with the dog’s signs.

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Symptoms of hypothyroidism in dogs

  • Tiredness, mental dullness and less tolerance of exercise.
  • Weight gain without eating more; seeking warm places.
  • Hair loss on the trunk that does not itch, a “rat tail”, darker or greasy skin.
  • Recurrent skin or ear infections; a slow heart rate.

These are the signs listed in the AAHA guidelines; nerve problems are less common [1]. The disease is usually caused by immune-mediated thyroiditis, and the average age at diagnosis is 6.8 years [1]. Beagles, Golden Retrievers, Dobermans and Great Danes are among the breeds at higher risk [1].

What routine blood work shows

Routine results are not specific. The most common findings are a high fasting cholesterol, high triglycerides and a mild non-regenerative anaemia [1]. In hypothyroid dogs, cholesterol can rise mildly to markedly; values above about 750 mg/dL have been associated with a risk of atherosclerosis [2].

The thyroid tests

TestIn hypothyroidismWhat to know
Total T4 (TT4)LowA value in the upper half of the range generally excludes hypothyroidism; a low value alone does not diagnose it [1].
Free T4 (fT4)LowLess affected by other illnesses; measured by equilibrium dialysis ideally [1].
TSHHighNormal in about 20–40% of hypothyroid dogs, so two of the three tests agreeing supports the diagnosis [1, 3].
Thyroglobulin autoantibodiesOften positiveShow thyroiditis; can falsely raise the T4 result [1].

How to read the T4 line

Here is an example line from a dog tested for hypothyroidism.

An annotated line from a lab report · Illustrative example
AbbreviationResultUnitLaboratory intervalFlag
TT4 Total thyroxine0.6µg/dL1.0–4.0L
  1. Abbreviation TT4, T4 or thyroxine. Free T4 appears as fT4.
  2. Result 0.6 µg/dL in this illustrative line: below the interval.
  3. Unit µg/dL in the United States, nmol/L in Europe (µg/dL × 12.87 = nmol/L).
  4. Laboratory interval An illustrative interval; each laboratory sets its own, and sighthounds normally run lower [1].
  5. Flag L: below the interval. Your vet checks for other illnesses and drugs, then confirms with free T4 and TSH [1, 3].

What can make a dog look hypothyroid when it is not

  • Other illnesses (“euthyroid sick syndrome”). Any significant illness lowers total T4, and free T4 in severe illness. Testing should wait until the dog has recovered [1].
  • Medicines. Corticosteroids, phenobarbital, some anti-inflammatory drugs, sulfonamide antibiotics, clomipramine, toceranib and others alter thyroid tests [3]. In one study, three weeks of a sulfonamide antibiotic lowered total T4 in half of the dogs [3].
  • Breed. Healthy Greyhounds, Salukis, Deerhounds, Alaskan sled dogs and Shar-Peis often have a total T4 below standard intervals [1].

Washout periods matter: for example, at least 6 weeks after stopping phenobarbital, and at least 4 weeks after stopping a sulfonamide antibiotic [3].

Related articles

Cushing’s disease in dogs

Another hormonal cause of hair loss and high cholesterol.

Read the article

En français : l’hypothyroïdie du chien.

Questions to ask your veterinarian

  1. Are free T4 and TSH needed to confirm the diagnosis?
  2. Could another illness or one of my dog’s medicines explain the low T4?
  3. Does my dog’s breed have different normal values?
  4. When should the T4 be rechecked after starting treatment, and at what time after the tablet?
  5. Which of the current signs should improve, and how quickly?
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Frequently asked questions

What are the signs of hypothyroidism in dogs?

Tiredness, weight gain, seeking warmth, non-itchy hair loss, a “rat tail”, recurrent skin or ear infections and a slow heart rate [1].

Is a low T4 enough to diagnose hypothyroidism?

No. Other illnesses, medicines and some breeds lower T4. The diagnosis rests on a low T4 and free T4 with a high TSH, read with the signs [1, 3].

What can trigger hypothyroidism in dogs?

Most cases come from immune-mediated damage to the thyroid. Some medicines, such as sulfonamide antibiotics, can cause a true but reversible hypothyroidism [1, 3].

Can hypothyroidism in dogs be reversed without medication?

Thyroid damage from thyroiditis does not reverse; treatment replaces the missing hormone, usually for life. Drug-induced changes can resolve after the drug is stopped [3].

What is a normal T4 for a dog?

Each laboratory sets its own interval, and sighthounds normally run lower. A T4 in the upper half of the interval generally excludes hypothyroidism [1].

Sources

  1. Bugbee A et al. 2023 AAHA Selected Endocrinopathies of Dogs and Cats Guidelines. J Am Anim Hosp Assoc. 2023;59(3):113-135. doi.org/10.5326/JAAHA-MS-7368
  2. eClinPath, Cornell University College of Veterinary Medicine. Cholesterol. eclinpath.com/chemistry/energy-metabolism/cholesterol
  3. Bolton TA, Panciera DL. Influence of medications on thyroid function in dogs: an update. J Vet Intern Med. 2023;37(5):1626-1640. doi.org/10.1111/jvim.16823

Laboratory methods and reference intervals vary. This article explains; it does not diagnose. For a decision about your pet, use the original report and talk to your veterinarian. Our editorial policy.

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