Hypothyroidism (Autoimmune Hypothyroiditis)

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Thank you to Hazel Fitzgibbon for providing this disorder information.

hypothyroidism

Summary

Autoimmune Hypothyroiditis (AH) is the most common thyroid disease in dogs covering 90% of hypothyroid cases.

Hypothyroid is used to describe the condition that occurs when the thyroid gland is not incorporating iodine into the hormones T3 (triiodothyronine) and T4 (thyroxine). A deficiency of these hormones may in turn affect many functions throughout the body.

Autoimmune Hypothyroiditis (AH) is the most common thyroid disease in dogs covering 90% of hypothyroid cases.

Hypothyroid is used to describe the condition that occurs when the thyroid gland is not incorporating iodine into the hormones T3 (triiodothyronine) and T4 (thyroxine). A deficiency of these hormones may in turn affect many functions throughout the body.

AH occurs when antibodies in the blood or tissue start to attack the thyroid gland, thus affecting the amount of T3 and T4 produced. Why they attack is still the source of mystery, but there is a genetic element involved in some breeds. Research is still ongoing in Samoyeds but researchers suggest that up to 50 breeds of dog may suffer from this condition.

Some clinicians assert that the phrase hypothyroidism should be given to the end stages of the disease, as a dog can have AH and not exhibit clinical signs for a long time.

Signs and Symptoms

Actual signs of the disease don’t start to appear until more than 75% of the Thyroid gland has been destroyed by antibodies. Signs are typically first seen at 2-5 years of age.

Because the thyroid is an important master gland, if it doesn’t work, it can affect multiple organs, from the skin to fertility. See Jean Dodd’s paper referenced below for a comprehensive list.

Classic signs are alopecia, greasy regrowth of hair especially on the body and the ‘rat tail’; lethargy, easy weight gain, not liking exercise. Also slow heart rate and sometimes severe neurological signs occur, with ataxia, circling and unusually for Samoyeds, aggression. Infertility in males and females may occur.

Causes

Most affected breeds are now thought to have a genetic component. It appears to be polygenic (has a number of different genes involved).

It has been suggested that nutrition and toxins may trigger a genetically susceptible animal.

Risk Factors

Related dogs with disorder.

Exposure to some toxins in genetically susceptible animal.

Diagnostic Tests

If hypothyroidism is suspected, a dog should have a medical history taken, a complete physical, and blood tests.

Note: Recent vaccinations, especially rabies7, or testing a bitch during estrus8 can interfere with test results.

Several blood tests are available for diagnosing hypothyroidism and AH. Testing has improved over the last few years. T4 and TSH levels used to be the only ones available for diagnosis. It is suggested that any dogs that have only had these tests and been declared clear be tested under the newer protocols.

  • T4 – The total amount of the main circulating thyroid hormone in serum.
  • Free T4 (FT4) – T4 that is not bound to serum proteins. This is the form of T4 that is available to enter cells.
  • TSH - Thyroid stimulating hormone; this does as it says and pushes the thyroid to produce T3 and T4. If the levels are elevated it means that not enough T3 and T4 are being produced.
  • TgAA - Thyroglobulin Autoantibodies: This last test is particularly revealing as these are the antibodies that destroy the thyroid.

The results of the testing depend on the cause and the progression of the disease. For T4, what is considered “normal” can also be breed-specific. The normal range for Samoyeds is currently under investigation.

The first 3 tests can determine if the dog is functionally hypothyroid, but do not indicate if the dog has the autoimmune version. That can only be determined with the TgAA test.

If the thyroglobulin autoantibody (TgAA) levels are elevated but the T4 levels still relatively normal it means that AH is confirmed but there is still enough functioning thyroid to produce the T4 and FT4. It is even possible that the affected dog has no symptoms. These tests should be repeated as the dog gets older. At that time, the TgAA may go back down, when there is no thyroid left for the antibodies to attack, but T4 may be low.

Interpreting these test results is a slippery slope and it is worthwhile to seek consultation with a Vet with experience in Endocrinology, as this needs to be carefully monitored. This is a complicated disorder and even with the best will in the world, misdiagnosing or incorrect dosing could prematurely ruin what is left of your pet’s thyroid system.

There are other things that can cause abnormal thyroid test results. For instance, there is a condition called compensatory hypothyroidism, or sick euthyroid syndrome (SES) which can be caused by a number of clinical conditions including trauma, stress, injury, illness, poor diet.10 Serum total T4, free T4, and total T3 concentrations may be low (ie, in the hypothyroid range), but the TSH is usually normal.11 The difference between SES and true hypothyroidism is that once the stressor is removed (ie illness, injury etc.), the thyroid gland returns to normal function11.

The T4 alone can be decreased for various reasons; toxins are one, but more likely is an adverse effect of medication. The following medications all can affect thyroid test results:

  • Glucocorticoids: high dosages, chronic therapy: T4 falls, FT4 more resistant, TSH is unchanged or decreased.
  • Sulfonamides: chronic therapy may cause reversible hypothyroidism and low T4
  • Phenobarbital: chronic therapy - mild fall in T4 and FT4; TSH remains normal in most dogs, but rises in a small percentage.
  • Carprofen: mild fall inT4, FT4 normal
  • Clomipramine: mild fall in T4 and FT4; TSH remains normal

It is important that current medication be taken into consideration when any dog is being diagnosed for any disorder, but thyroid levels seem to be very prone to deviation.

Treatment Guidelines

Note: Treatment of animals should only be performed by a licensed veterinarian. Veterinarians should consult the current literature and current pharmacological formularies before initiating any treatment protocol.

There is very good medication available which can control the disorder for many years.

Levothyroxine, a synthetic form of T4 sold as Soloxine or Synthroid, is the drug of choice for most clinicians; it is given once or twice a day and dosage is dependent on the weight of the dog. Another T4 test should be done after a fortnight to check that the correct dosage is being given. T4 and FT4 should also be rechecked at least once a year. The timing of when the blood sample is taken vs when the medication is given can be critical to accurate results, so check with your veterinarian.

Where the affected animal has shown clinical obesity on diagnosis, weight should start to drop off as a result of medication. He or she should be reweighed regularly to make sure you are not overdosing.

Management

According to the OFA and numerous published reports, autoimmune hypothyroiditis (AH) is a heritable condition. The TgAA assay is recommended to screen dogs who will be used for breeding because this assay can detect the disease before any clinical symptoms occur4. The best age for testing for TgAA is between ages 2 and 6.

References

General:

Dr Jean Dodds has a worldwide reputation on this subject. This is an overview:

References - Diagnostic Testing and Interpretation:

This paper explains the difficulties of testing for thyroid problems:

A bit technical, but this is the best and most useful paper on diagnostics (you will have to register and login to access this article):

  • (6) Ferguson, D.C. Update on Thyroid Diagnostic Tests for Dogs. In: ACVP and ASVCP (Eds.), 56th Annual Meeting of the American College of Veterinary Pathologists (ACVP) and 40th Annual Meeting of the American Society for Veterinary Clinical Pathology (ASVCP), 2005 - Boston, MA, USA. Ithaca: International Veterinary Information Service 2 bones (Available at: www.ivis.org Document No. P2202.1205.)

Information from a company that does diagnostic testing:

  • (7) Assessing Thyroid Function from Antech Diagnostics (link no longer available) 2 bones

Information from the OFA on how they interpret test results:

References - Other:

Description of Sick Euthyroid Syndrome:

1 bone 2 bones 3 bones 4 bones (full description of ratings)

The OFA registry has general information and contains the requirements of registering with that organization:

OFA Website 3 bones

OFA Thyroid Application

1 bone 2 bones 3 bones 4 bones (full description of ratings)

SCARF-Sponsored Research

  • AKCCHF Grant #00305: Histocompatibility Alleles Conferring Susceptibility to Canine Diabetes, Immune-Mediated Thyroiditis and Immune-Mediated Hemolytic Anemia. Wayne Potts, PhD, University of Utah.
  • AKCCHF Grant #00372: The Establishment of Breed-Specific Reference Intervals for Thyroid Hormones. Rebecca L Davies, PhD; Sheila Torres, DVM MS PhD DACVD.

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