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Euthyroid Sick Syndrome

Euthyroid sick syndrome (ESS), also known as sick euthyroid syndrome, refers to the condition in dogs in which concurrent nonthyroidal illness (NTI) 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 ESS can be misleading and result in an erroneous diagnosis of hypothyroidism. Although the exact mechanism of this syndrome is unknown, it is believed to be a protective mechanism of the body during NTI to decrease cellular demands by slowing metabolism.

Euthyroid sick syndrome can be associated with endocrine disorders such as hyperadrenocorticism, hypoadrenocorticism, and diabetes mellitus, as well as any significant organ disease, particularly renal failure and liver disease. Cardiac failure, hemolytic anemia and infectious disease are also etiologies of the ESS.

Differentiating hypothyroid from euthyroid dogs can be challenging and requires evaluation of more than a single thyroid function test. Discordant results on a thyroid profile such as a normal TSH in combination with a low TT4 or fT4 may be attributable to the 15-25% of hypothyroid dogs that have a normal TSH values, or it may be a euthyroid dog either with concurrent illness (ESS) or without concurrent illness, particularly in sighthounds. In euthyroid dogs, the degree to which serum T4 and fT4 concentrations are decreased is directly correlated with the severity of nonthyroidal disease. Concomitant increases in TSH occur less commonly, even in severe NTI.

Frequently the differentiation of ESS from true hypothyroidism is based on a combination of factors including signalment, history, clinical signs and clinical pathology results. Hypothyroidism typically occurs in middle-aged and mid to large-breed dogs. History and physical exam changes that are not consistent with hypothyroidism may actually point to a nonthyroidal illness. In the absence of characteristic biochemistry and CBC changes associated with hypothyroidism such as nonregenerative anemia and hypercholesterolemia, the diagnostician prudently investigates other etiologies for the clinical pathology changes noted. A dog with normal TSH but severe suppression of TT4 and fT4 and signs of severe illness is more likely to have ESS than true hypothyroidism.

Recent studies evaluating thyroid ultrasonography have shown promise in measuring thyroid volume and relative echogenicity to distinguish euthyroid glands from hypothyroid glands, and 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
Kantrowitz LB et al. Serum total thyroxine, total triiodothyroinine, free thyroxine, and thyrotropin concentrations in dogs with nonthyroidal disease. JAVMA 2001; 219(6):765-769.

Peterson ME, Melian C, and Nichols R. Measurement of serum total thyroxine, triiodothyroinine, free thyroxine, and thyrotropin concentrations for diagnosis of hypothyroidism in dogs. JAVMA 1997; 211(11): 1396-1402.

Bromel C et al. Ultrasonographic evaluation of the thyroid gland in healthy, hypothyroid, and euthyroid golden retrievers with nonthyroidal illness. J Vet Intern Med 2005;19:499-506.

Reese S et al. Thyroid sonography as an effective tool to discriminate between euthyroid sick and hypothyroid dogs. J Vet Intern Med 2005;19:491-498.

Nelson and Feldman. Canine and Feline Endocrinology and Reproduction. Third Edition. Elsevier Science (USA). 2004.