The Diagnosis of Thyroid Disease in Dogs

By Jackie Casey JP, BVetMed, MRCVS

We are frequently requested to perform diagnostic testing for thyroid disease in dogs looking to assess thyroid status. Sometimes the results are easy to interpret; others are more of a challenge.

Adding ‘thyroid’ tests to a routine panel when there are no overt clinical suggestions of hypothyroidism can lead to complex problems with interpretation. The first most overriding principle before checking thyroid status is ‘does the dog have any clinical signs suggesting hypothyroidism?’

These commonly include:

  • Metabolic changes: Fatigue, lethargy, mental dullness, weight gain, skin and urinary tract infections due to depressed immunity.
  • Skin changes: seborrhoea sicca or oleosa, and comedomes. Failure of hair regrowth, and cycling, leading to symmetrical alopecia, or changes in coat quality/colour. Poor wound healing, hyperpigmentation, recurrent infections due to poor immunity.
  • Neuromuscular signs: peripheral neuropathy, weakness, (rarely myxoedema coma)
  • Cardiovascular signs: atherosclerosis, bradycardia, decreased contractility, low voltage QRS
  • Ocular signs: vascular lipidaemia
  • Reproductive changes: loss of libido, infertility, prolonged interoestrus period
  • Gastrointestinal signs: constipation, megaoesophagus

Please note PU/PD is not amongst these!

Thyroid disease in dogs

The more of the various clinical signs that are manifesting in the dog the more accurate the interpretation of the test results becomes. Hypothyroidism is a slow onset disease and clinical signs can be vague and attributable to many other conditions. If hypothyroidism is suggested by clinical presentation there are several tests which can be performed.

Total T4. Inexpensive, widely available, easily measured

Non thyroidal illness NTI and certain drugs (anticonvulsants, steroids, potentiated sulphonamides, and some NSAIDs) will also depress the circulating Total T4 concentration, and age has an effect of lowering the TT4 value. Sighthounds will also have low values. Total T4 in the normal range will tend to rule out hypothyroidism, however it must be remembered that approximately 10% of hypothyroid dogs will have T4 autoantibodies which will falsely elevate the TT4 result. This is where the index of suspicion on your clinical presentation would suggest further investigation is required when the Total T4 result is within the normal range.

TSH. Should not be measured alone

Decreased circulating thyroid hormones should cause an increase in TSH levels. However roughly 25% of hypothyroid dogs will have reference range TSH value. It has also been found that elevated TSH values can be seen in euthyroid dogs recovering from non thyroidal illness and in early hypothyroidism making this test on its own of limited value.

Free T4

Less affected by NTI and drugs, more specific for hypothyroidism, more expensive, less widely available, can still be normal in early hypothyroidism, can be affected by NTI/drugs. This is the metabolically active form of T4 and reflects more closely the thyroidal status than Total T4. It is less affected but NTI and drug therapies, and severe illness will be required to depress FT4 values. FT4 must be measured by equilibrium dialysis to give accurate results.

Thyroglobulin autoantibodies TgAA.

Negative result does not rule out hypothyroidism, and no assessment of thyroid function Measurement confirms the presence of lymphocytic thyroiditis – but this is not the pathology present in all dogs (some studies suggest 50% of cases), and levels decline with decreased thyroid tissue so is only helpful in early cases. Some breeders are using this as a screening test in young dogs of breeds known to be predisposed to hypothyroidism. In such cases TGAA can be used as a marker of thyroid disease in dogs which are at present clinically healthy.

No one test is the ‘Gold Standard’ and will give the correct answer in all cases.

Combining tests can help to counteract each individual test’s limitations.

Generally a Total T4 and cTSH should be considered the initial screening profile.

A low Total T4 and high cTSH gives a diagnosis of hypothyroidism in most cases, and can be performed easily and economically. If there are any indications of NTI being the cause for a decreased TT4 then Free T4 (ED) and TgAA assays will further investigate thyroid status.

Historically thyroid status was investigated with either TRH or TSH stimulation tests, but again these did not always give a definitive answer and they have largely been replaced by the less time consuming and expensive tests mentioned above.