Euthyroid Sick Syndrome

Euthyroid sick syndrome is a condition in dogs in which concurrent non-thyroidal illness causes suppression of serum concentrations of circulating thyroid hormone without true pathology of the thyroid gland.

Decreased concentrations of T4, and to a lesser degree fT4, with euthyroid sick syndrome can be misleading and result in an erroneous diagnosis of hypothyroidism. The exact mechanism is unknown, but it is believed to be a protective mechanism to decrease cellular demands by slowing metabolism.

Euthyroid sick syndrome can be associated with:

• Endocrine disease
  • hyperadrenocorticism
  • hypoadrenocorticism
  • diabetes mellitus
• Any significant organ disease
  • renal failure
  • hepatic failure
  • cardiac failure
• Hemolytic anemia
• Infectious disease

The degree to which serum T4 and fT4 concentrations are decreased is directly correlated with the severity of non-thyroidal disease. Concomitant increases in TSH occur less commonly, even in severe non-thyroidal illness.

Differentiating hypothyroid from euthyroid dogs can be challenging and requires evaluation of more than a single thyroid function test.

Thyroid ultrasonography, for the measurement of thyroid volume and relative echogenicity to distinguish euthyroid glands from hypothyroid glands, has shown some promise. This may prove to be the diagnostic tool veterinarians have been looking for to confirm hypothyroidism in cases of discordant thyroid profile results.

Suggested Additional Reading

1. Brömel C, Pollard RE, Kass PH, Samii VF, Davidson AP & Nelson RW. (2005) Ultrasonographic evaluation of the thyroid gland in healthy, hypothyroid, and euthyroid golden retrievers with non-thyroidal illness. J Vet Intern Med 19:499-506.

2. Kantrowitz LB, Peterson ME, Melián C & Nichols R. (2001) Serum total thyroxine, total triiodothyroinine, free thyroxine, and thyrotropin concentrations in dogs with non-thyroidal disease. J Am Vet Med Assoc 219:765-769.

3. Nelson RW & Feldman EC. (2004) Hypothyroidism In Canine and Feline Endocrinology and Reproduction. 3rd Edition. Saunders, St Louis, MO, USA.pp 86-142.

4. Peterson ME, Melian C & Nichols R. (1997) Measurement of serum total thyroxine, triiodothyroinine, free thyroxine, and thyrotropin concentrations for diagnosis of hypothyroidism in dogs. J Am Vet Med Assoc 211:1396-1402.

5. Reese S, Breyer U, Deeg C, Kraft W & Kaspers B. (2005) Thyroid sonography as an effective tool to discriminate between euthyroid sick and hypothyroid dogs. J Vet Intern Med 19:491-498

Top of page