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Confirmed cases of hypothyroidism are very rare. Although thyroid dysfunction has been commonly referred to in texts and there is a copious literature on equine hypothyroidism, the evidence for its existence in adult horses is tenuous. The clinical syndromes attributed to it are based only on experimental cases in which both thyroid glands have been removed (Hillyer & Taylor 1997).

Many of the horses in which hypothyroidism is diagnosed have a metabolic syndrome that is now well recognized. There is no convincing evidence that thyroid deficiencies cause laminitis, fertility problems, reduced performance, anaemia or anhidrosis (see p. 344). There are, likewise, few convincing cases of skin disease associatedwith adult onset hypothyroidism; only a few cases have been reported in adult horses (Hillyer & Taylor 1992). Bilaterally symmetrical alopecia occurred in one case (Shearer 1988).

Key points: Hypothyroidism

1. Many metabolic derangements are blamed on thyroid gland dysfunction. In adult horses hypothyroidism is extremely rare and any cutaneous signs are vague and ill defined. In foals born to mares with an abnormally high iodine intake, severe musculoskeletal deformities occur. These may also show skin signs.

2. In foals severe musculoskeletal deformities and goitre and sparse, silky soft (almost premature) velvet coat are present. The hoof may be abnormally soft and rubbery. There may be some seborrhoea. The dermatological signs are usually irrel-evant. Adult horses could present as bilaterally symmetrical alopecia that responds to thyroid supplementation.

3. Diagnosis in foals is based on the presence of a visible/palpable goitre and musculoskeletal deformities. By the time the foal is 24 hours old dynamic or static thyroid hormone tests are probably of no value. In adult horses dynamic thyroid function tests can be used but interpretation is highly problematic.

4. Treatment of foals is ineffective – supplemental thyroid hormones do nothing to help. In adults the effects of even high doses of thyroid replacement therapy are almost imperceptible.

5. The prognosis for foals is hopeless although mildly affected foals may survive. In adults the outlook is guarded simply because nothing much is known about the condition.

Foals born in areas where iodine deficiency occurs have been reported to have signs that can be directly attributed to hyperiodic hypothyroidism and this syndrome is well recognized (Irvine & Evans 1977).

Clinical signs

Diagnostic confirmation

Treatment

Oral L-triiodothyronine (4 mg/kg bodyweight) is given daily for at least 10 days. Treatment course may require months or years or may show no response at all.

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Equine pituitary pars intermedia dysfunction (PPID/Cushing’s disease) is almost always due to the very common hyperplasia of the pars intermedia of the neurohypophysis. It occurs commonly in horses and ponies over 12–15 years of age. The condition is very rare in animals under 7–10 years of age.

It is likely that the pathophysiology relates to a brainstem degeneration and a consequent loss of inhibitory control on the pars intermedia. There is a reduction in dopamine secretion which normally inhibits the brainstem melanotropes. The pars intermedia gradually undergoes functional hypertrophy with increasing release of adrenocorticotropin. This in turn results in hypersecretion from the adrenal cortex. Excessive endogenous cortisol has profound and widespread effects on vascular endothelia, glucose and fat metabolism, immune responses, wound healing and endocrine secretions. These effects are used to explain the laminitis, fat redistribution, weight loss, immunosuppression, hyperglycaemia and polydipsia. This is a plausible (if over-simplified) course of events to explain the corticosteroid-related abnormalities but it does not really explain all the clinical signs and the behavioural changes, blindness, seizures and hirsutism in particular. These may be attributable to brainstem alterations and to the space-occupying nature of the hypertrophic pituitary gland in the brainstem and hypothalamus in particular.

No breed, sex or colour predisposition has been established, but ponies and geldings may be over-represented (possibly because they are commonly kept to older ages than larger horses). Tumours may be very small (almost invisible) but functional while others may be large enough to cause physical effects but have little physiological effect (Hillyer et al 1992).

Key points: Pituitary pars intermedia dysfunction

1. A very common geriatric condition of horses over the age of 15 years resulting from hyperplasia of the pars intermedia of the pituitary gland.

2. Clinical signs reflect a wide range of neurological and endo-crine abnormalities and include hirsutism, weight loss, sweat-ing, laminitis and fat redistribution. Impaired immunity results in opportunistic skin infections including bacterial dermatitis and abscessation, fungal infections and ectoparasites (mites and lice) and endoparasites.

3. Diagnosis can be confirmed using dynamic endocrine tests or cortisol/insulin concentrations and urinary excretion ratios. Laboratory tests confirm hyperglycaemia, hypercortisolaemia and often mild increases in tissue/organ enzymes.

4. Treatment is palliative and centred around anti-serotonin or dopaminergic compounds administered orally. Direct sup-pression of the adrenal gland is not usually effective. Nursing measures such as careful dental care, clipping of the coat and routine farriery and anthelmintic treatment are essential.

5. The prognosis is guarded but for those horses with few major complications such as laminitis and severe weight loss, the outlook is quite good. Survival for some years can be expected if medication can be sustained

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