You are asked to examine an 18-year-old Warmblood gelding that has developed progressive alopecia, which started in the neck region. Haematological evaluation reveals a haematocrit of 30% (n = 36–42%) and a normal leucocyte count (8.9x109/l with 21% lymphocytes, 1% eosinophils and 78% neutrophils). The total serum protein concentration is normal (81g/l with 31.7% albumin, 14.3% alpha-globulins, 29.4% beta-globulins and 24.6% gamma-globulins). Basal T4 concentration is low.
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What is the most likely endocrinopathy to cause these lesions?
Hypothyroidism. Thyroid dysfunction is rare in the horse. Equine hypothyroidism can be classified into four groups:
hypothyroidism in foals;
hypothyroidism in adult horses;
euthyroid sick syndrome; and
thyroid neoplasia.
Hypothyroidism in foals is usually associated with prognathism, ruptured tendons, forelimb contracture, delayed ossification of the carpal and tarsal bones, and goitre. The prognosis is extremely poor.
The clinical signs of hypothyroidism in adult horses are cessation of growth, hypothermia, lethargy, irregular areas of alopecia and thickened face.
Which three thyroid hormones can you name, and how are they controlled?
Thyroglobulin is the prohormone of the thyroid hormones
thyroxine (T4),
tri-iodothyronine (T3) and
reverse triiodothyronine (rT3).
Once released into the circulation, the hormones are rapidly bound to plasma proteins and only 0.2% T3 and 0.06% T4 is free or unbound. Only the free forms are biologically active.
Thyroid-releasing hormone (TRH) is released from the hypothalamus in response to decreased free T4, which in turn stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH).
There is a slight diurnal variation in thyroid hormone concentrations with low concentrations during the night.
Thyroid hormones regulate cell growth and differentiation, and regulate energy metabolism.
Does diet influence the plasma concentrations of these hormones?
In weanling foals fed a diet in accordance to their energy and protein demands, both serum T4 and T3 concentrations increase within two hours of feeding.
Undernutrition in foals results in an increase in T4 concentration, but T3 concentration remains unchanged.
On the other hand, excess energy and protein in foals result in a decreased T4 concentration and an increased T3 concentration.
In adult horses, food restriction induces a decrease in thyroid hormones.
In addition, non-thyroidal factors affect thyroid function in horses.
Three of these factors,
age,
pregnancy and
low temperature,
are associated with increased levels of thyroid hormones.
Thyroid hormone concentrations in normal foals are up to 14 times higher compared with those of horses older than two years.
Serum thyroid hormone concentrations are decreased following concurrent disease and the administration of phenylbutazone (T4 only).
However, the administration of dexamethasone does not change baseline T3 and T4 concentrations.
Disease unrelated to thyroid disease can depress basal thyroid hormone levels as a normal response to minimize the catabolic effects of thyroid hormone during disease.
In cases of euthyroid sick syndrome, low baseline T3 and T4 concentrations respond normally to TSH administration.
