The term euthyroid sick syndrome is used to describe the condition in animals which, in response to a pathological process, have a decrease in levels of circulating thyroid hormones despite an apparently normal thyroid gland. In these subjects, the reduction in the serum concentrations of total thyroxine (T4) and free T4 (fT4) may be due to decreased secretion of thyroid-stimulating hormone (TSH) secondary to suppression of the hypothalamic-pituitary axis, reduced synthesis of T4, decreased availability/binding affinity with plasma proteins, inhibition of peripheral deiodination of T4 to triiodothyronine (T3) or to a combination of these factors. It is thought that the reduction in levels of circulating thyroid hormones in many cases of euthyroid sick syndrome is an adaptation/defence mechanism raised by the body in order to reduce cell metabolism during a period of illness.
In dogs with euthyroid sick syndrome the circulating levels of T4 are often between 6.5 and 13 nmol/l although in more severe cases (for example, in animals with anaemia or cardiomyopathy) they may be below 6.5 nmol/l. The concentrations of fT4, obtained by dialysis, may be reduced, but usually to a lesser degree than those of T4. Depending on the underlying pathological process, the concentrations of canine TSH, may be normal or, in some cases, even increased to exceed values of 1.0 ng/dl. In any case, TSH is much less influenced by euthyroid sick syndrome than are T4 and fT4.
From a therapeutic point of view it is important to detect the presence of euthyroid sick syndrome and differentiate it from a true state of hypothyroidism. Although there are no reports of harmful consequences of erroneous administration of levothyroxine to dogs with euthyroid sick syndrome, supplementation with thyroid hormones is not recommended in dogs with this condition.
Systemic diseases are the conditions that most frequently induce euthyroid sick syndrome in dogs; the degree of reduction of the thyroid hormones is directly related to the severity of the underlying disease. Such diseases include liver failure, kidney failure, heart disease, septicaemia, immune-mediated disorders, hypercortisolism (spontaneous and iatrogenic), hypoadrenocorticism and diabetic ketoacidosis, as well as anaesthesia and surgery. In these cases it is even more difficult to make a definitive diagnosis and it is, therefore, worth using specific diagnostic tests such as stimulation with TSH rather than relying on a single parameter of thyroid function.
Suggested readings
- Feldman E.C. , Nelson R.W.: Hypothyroidism. In:Canine and Feline Endocrinology and Reproduction, III edizione Saunders, pg. 87-151, 2004.
- Kantowits L.B. et al.:Serum total thyroxin, total triiodothyronine, free thyroxin, and thyreotropin concentrations in dogs with nonthyroidal illness. J Am Vet Med Ass. 219, (6), 1999.