Hypothyroidism

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Hypothyroidism is a rare autosomally recessive hormonal disease which can occur in any breed of cats[1] characterised by lethargy, alopecia and weight gain.

A hereditary form of hypothyroidism (dwarfism) has been reported in related Abyssinian cats, but may not be breed specific. Homozygous cats had reduced growth rate, shorter stature, kitten-like features, constipation and goiter. Hypothyroidism can also occur iatrogenically as a result of treatment of hyperthyroidism[2].

In hypothyroidism, impaired production and secretion of the thyroid hormones result in a decreased metabolic rate.

In cats, iatrogenic hypothyroidism is the most common form caused by aggressive treatment of hyperthyroidism[3], although concurrent illness can cause laboratory values for T4 to mimic hypothyroidism[4]. Hypothyroidism develops in these cats after treatment for hyperthyroidism with radioiodine, surgical thyroidectomy, or use of an antithyroid drug.

Although naturally occurring hypothyroidism is an extremely rare disorder in adult cats, congenital or juvenile-onset hypothyroidism does also occur. Recognized causes of congenital hypothyroidism in the cat include intrathyroidal defects in thyroid hormone biosynthesis (dyshormonogenesis), an inability of the thyroid gland to respond to TSH, and thyroid dysgenesis. All reported cats with hypothyroidism have had the primary (thyroidal) disorder. Secondary (pituitary) or tertiary (hypothalamic) hypothyroidism have not been described in either the juvenile or adult cat.

In adult cats, clinical signs associated with hypothyroidism usually include lethargy, dullness, non-pruritic seborrhoea sicca, hypothermia, otitis externa and occasionally bradycardia.

Obesity may develop, especially in cats with iatrogenic hypothyroidism, but it is not a consistent sign. Bilaterally symmetric alopecia, with the exception of pinnal involvement, does not appear to develop, but focal areas of alopecia over the craniolateral carpi, caudal hocks, and dorsal and lateral tail base have occasionally been observed.

In young cats with congenital or juvenile-onset hypothyroidism, the clinical signs include disproportionate dwarfism, severe lethargy, mental dullness, constipation, and bradycardia.

Diagnosis is usually confirmed by assessment of blood T4 levels.

The determination of basal serum total T4 concentration by radioimmunoassay techniques may provide important information to rule out a diagnosis of hypothyroidism. Because T4 is produced only by the thyroid gland, hypothyroid animals can, in most cases, be distinguished based on a low resting serum total T4 concentration. However, many nonthyroidal illnesses and administration of various drugs including glucocorticoids, sulfonamides, anticonvulsants (eg, phenobarbital), NSAID, and radiocontrast agents may “falsely” lower serum T4 concentrations. Even when historical and physical findings do not suggest other factors that would lower serum T4, the diagnosis of hypothyroidism is best confirmed by measuring free T4 concentration (by dialysis), which is affected to a much lesser degree by nonthyroidal illness or drug therapy than is the total T4 concentration.

Thyroxine (T4) is the thyroid hormone replacement compound of choice. With few exceptions, replacement therapy is necessary for the remainder of the amimal’s life; careful initial diagnosis and tailoring of treatment is essential. The reported replacement dosages for Levothyroxine (SyntheticT4) in cats is 20-60 μg/kg daily.

The most important indicator of the success of therapy is clinical improvement.

References

  1. Blois SL et al (2010) Use of thyroid scintigraphy and pituitary immunohistochemistry in the diagnosis of spontaneous hypothyroidism in a mature cat. J Feline Med Surg 12(2):156-160
  2. van Hoek IM et al (2010) Thyroid stimulation with recombinant human thyrotropin in healthy cats, cats with non-thyroidal illness and in cats with low serum thyroxin and azotaemia after treatment of hyperthyroidism. J Feline Med Surg 12(2):117-121
  3. Williams TL et al (2010) Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism. J Vet Intern Med 24(5):1086-1092
  4. Blois, SL et al (2010) Use of thyroid scintigraphy and pituitary immunohistochemistry in the diagnosis of spontaneous hypothyroidism in a mature cat. JFMS 12:156-160