Page 930 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Endocrine system 905

VetBooks.ir Prognosis when evaluating a foal with suspected thyroid
disease. Selenium is a cofactor in the enzymatic
The prognosis for adult horses with hypothyroidism
process that iodinates proteins to make thyroxine
is good if they are properly diagnosed and treated.
in the thyroid gland, and selenium deficiency has
THYROID DISEASE IN THE FOAL been suggested as potential goitrogen in the horse.
Iodine-trapping plants such as mustard may also
Definition/overview cause goitre. Excessive iodine ingestion by the
A syndrome of hypothyroidism and goitre has been mare (e.g. kelp supplements) can cause goitre and
observed in foals exposed to an excess or deficiency hypothyroidism in neonatal foals while the mares
of iodine in utero. Another syndrome of hypothy- remain asymptomatic.
roidism associated with thyroid gland hyperpla-
sia and various skeletal abnormalities has been Clinical presentation
described mainly in foals born in Western Canada Clinical signs in neonatal foals born to mares ingest-
and the USA. The aetiology of this second syndrome ing excessive iodine during gestation include goi-
is unknown but factors such as nitrate, low iodine, tre, incoordination, hypothermia and poor suckle
low selenium or goitrogenic plant ingestion by the and righting reflexes. Clinical signs of a syndrome
mare have been suggested. of hypothyroidism described in foals in Western
Canada and the USA primarily involve congenital
Aetiology/pathophysiology musculoskeletal abnormalities, including mandibu-
Serum concentrations of thyroid hormones are lar prognathia (Figs. 6.1, 6.2), flexural deformities
much higher in neonates than they are in adults. of the forelimbs, ruptured digital extensor tendons
It is thought that these high concentrations are and incomplete ossification of the carpal and tarsal
important for perinatal organ system growth and bones (Figs. 6.3, 6.4). At birth, serum concentra-
maturation. Serum concentrations decrease grad- tions of thyroid hormones may be normal in these
ually after birth, with free concentrations enter- foals. It is thought that the musculoskeletal problems
ing the adult reference interval within the first are caused by hypothyroidism during key develop-
few weeks of life, and total concentrations reach- mental stages in utero.
ing adult reference intervals when foals are around
1 month old. It is important to be aware of the Differential diagnosis
relatively higher thyroid hormone concentrations Differential diagnosis in a weak foal with hypothy-
in foals and not use adult horse reference intervals roidism should include dysmaturity and sepsis.

6.1 6.2

Figs. 6.1, 6.2 Photograph (6.1) and lateral skull radiograph (6.2) of a foal with congenital hypothyroidism.
Note the mandibular prognathia.