Endocrine Endocrine

Thyroid

Serum T4 concentrations decrease by approximately 0.07 mg/100 ml per year in dogs with advancing age (Belshaw and Rijnbeck, 1979; Weller et al. 1983). If this fall has a significant effect thermoregulatory problems (hypothermia), and cardiovascular disturbances such as bradycardia or impaired myocardial contractility might be expected to result. Older animals might also be expected to have a reduced metabolic rate and a predisposition to develop obesity, which they do.

Hypothyroidism is relatively common in older dogs and may be associated with concurrent obesity. In some of these animals anaesthesia will be complicated by both the hypothyroidism and hypoinsulinaemia or insulin resistance. Hypothyroid animals are more susceptible to develop hypothermia and the vasodilatory effects of agents such as acepromazine and halothane may induce profound hypotension.



Adrenal

Aldosterone responses decrease in humans with advancing age and these changes are thought to be secondary to reduced renin secretion from the juxtaglomerular apparatus in the kidneys. In view of the high incidence of renal pathology in old animals it is reasonable to assume that such a decline might also occur in animals.

It has been suggested that corticosteroids should be administered to geriatric animals during prolonged periods of stress, surgery or anaesthesia to counter 'adrenal exhaustion'.

Hyperadrenocorticism (Cushing's syndrome) is most common in middle-aged to old dogs. It causes muscle weakness, reduced expiratory reserve volume, reduced chest wall compliance, increased blood volume and increased systolic and diastolic blood pressures (Feldman & Nelson 1987).

Adrenaline concentrations may increase, particularly in the presence of major organ system failure such as congestive heart failure. Plasma nor-adrenaline concentrations increase with age due to reduced clearance, but receptors compensate by becoming less sensitive.



Pancreas

Glucose tolerance deteriorates with advancing age and may be associated with hypoinsulinaemia (diabetes mellitus) or the development of peripheral insulin resistance. The administration of fluids containing glucose needs to be carefully considered in such patients, particularly if nutritional support is going to be given by total parenteral nutrition (TPN) when 50% dextrose solutions may be advocated. Chronic diabetics may have abnormal serum electrolyte concentrations which should be corrected before surgery.

In the presence of hyperinsulinaemia, hypoglycaemia may be precipitated during general anaesthesia and, as even a transient hypoglycaemia may cause brain damage, blood (and in some cases urine) glucose concentrations should be monitored during anaesthesia.