Page 931 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

906 CHAPTER 6

VetBooks.ir 6.3 6.4

Fig. 6.3 Dorsopalmar
radiograph of incomplete
ossification of cuboidal bones
of the carpus in a foal with
congenital hypothyroidism.

Fig. 6.4 Lateral radiograph
of incomplete ossification of
cuboidal bones of the tarsus
in a foal with congenital
hypothyroidism.

Diagnosis be necessary to prevent foals from knuckling or to
Diagnosis requires observation of a goitre and mea- prevent collapse of the cuboidal bones on the medial
surement of serum concentrations of thyroid hor- sides of the joints. If used, splints must be applied
mones (resting thyroid hormone concentrations are carefully and monitored closely. Foals wearing
much higher in neonates than in adults). If function splints may require help getting up to nurse and are
testing is performed by administering TRH, the at risk for developing decubital ulcers.
response in T values is more consistent than T in
3
4
neonatal foals. Prognosis
The prognosis for hypothyroid neonates is guarded.
Management For foals born with goitre, the prognosis improves if
Ensuring proper iodine concentrations in diets the foal can survive the first week of life. The prog-
fed to pregnant mares can prevent neonatal goitre. nosis for foals with musculoskeletal abnormalities
Thyroid hormone supplementation may be help- depends on the severity of the abnormalities. Certain
ful in the treatment of hypothyroid foals in terms problems, such as mandibular prognathia, are unlikely
of body temperature regulation, suckle reflex and to correct. Incompletely ossified cuboidal bones will
mental alertness. Since T must be converted to T ossify in time, but long-term prognosis depends on
4
3
for biological activity, there is a greater lag period whether the bones collapse before ossification occurs.
between administration and metabolic effect for T
4
compared with T . Therefore, a combination of T HYPERTHYROIDISM
3
3
and T supplementation would theoretically be more
4
beneficial to a neonate, but appropriate dosing is Hyperthyroidism is extremely rare in horses. There
unclear. Current dosing recommendations are listed have only been a few cases of naturally-occurring
in Table 6.2. Since the cause of the Western syn- hyperthyroidism in adult horses properly docu-
drome of thyroid dysfunction in foals is unknown mented in the literature, and these have been caused
at this time, preventive measures are also unknown. by thyroid hormone-producing tumours. Reported
Exercise should be restricted in foals born with clinical signs in these horses include weight loss,
incompletely ossified cuboidal bones and/or rup- tachycardia, tachypnoea, hyperactive behaviour,
tured extensor tendons. Lightweight splints may ravenous appetite and cachexia. Circulating thyroid