Page 768 - Small Animal Internal Medicine, 6th Edition

PART SIX Endocrine Disorders
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PART VI Endocrine Disorders
Richard W. Nelson and Ann-Marie Della Maggiore

VetBooks.ir CHAPTER 46

Disorders of the

Hypothalamus and

Pituitary Gland

POLYURIA AND POLYDIPSIA and usually have a psychogenic or behavioral basis for the
compulsive water consumption (see the discussion of psy-
Water consumption and urine production are controlled by chogenic PD, p. 7). A complete discussion of the diagnostic
complex interactions between plasma osmolality, fluid approach to PU-PD is presented in Chapter 38. An index
volume in the vascular compartment, the thirst center, the of suspicion for most of the endocrinopathies that cause
kidney, the pituitary gland, and the hypothalamus. Dysfunc- PU-PD can be raised after a review of the history, physical
tion in any of these areas results in the clinical signs of examination findings, and results of a complete blood count
polyuria (PU) and polydipsia (PD). In dogs normal water (CBC), serum biochemistry panel, serum thyroxine (T 4 )
intake is usually less than 80 mL/kg of body weight/24 h. concentration (cat), urinalysis, and urine culture. Specific
Water intake between 80 and 100 mL/kg/24 h is suggestive tests may be necessary to confirm the diagnosis (Table 46.1).
of PD but may be normal in some dogs. Water intake greater See the appropriate chapters in this section for a more com-
than 100 mL/kg/24 h confirms PD. Similar values are used plete discussion of the diagnosis and treatment of each of
for cats, although most cats drink considerably less than these endocrinopathies.
these amounts. Normal urine output ranges between 20 and Occasionally, physical examination findings and initial
45 mL/kg/24 h (1-2 mL/kg/h). PU in the dog and cat has blood and urine test results are normal in dogs and cats with
been defined as urine production greater than 50 mL/ PU and PD. Differential diagnoses in these dogs and cats
kg/24 h, although it is possible for urine production to be include diabetes insipidus, psychogenic PD, hyperadreno-
abnormal within the limits of these normal values in indi- corticism, mild renal insufficiency without azotemia, mild
vidual dogs and cats. hepatic insufficiency (most notably with portosystemic
A variety of metabolic disturbances can cause PU-PD. shunts), and the early stages of hypoadrenocorticism. Hyper-
Primary polyuric disorders can be classified on the basis of adrenocorticism, hypoadrenocorticism, renal insufficiency,
the underlying pathophysiology into primary pituitary and hepatic insufficiency should be ruled out before diag-
(central) and nephrogenic diabetes insipidus (NDI), second- nostic tests for diabetes insipidus or psychogenic PD are
ary NDI, osmotic diuresis-induced PU, and interference performed. Diagnostic tests to consider include evaluation
with the hypothalamic-pituitary secretion of arginine vaso- of the range of urine specific gravities obtained from several
pressin (AVP). The most common form of diabetes insipidus urine samples (discussed in the next paragraph), tests for
is acquired secondary NDI. This form includes a variety of hyperadrenocorticism (e.g., urine cortisol/creatinine ratio,
renal and metabolic disorders in which the renal tubules lose low-dose dexamethasone suppression test), hypoadrenocor-
the ability to respond adequately to AVP. Most of these ticism (e.g., baseline serum cortisol concentration), liver
acquired forms are potentially reversible after elimination of function tests (e.g., measurement of preprandial and post-
the underlying illness. prandial bile acid concentrations), determination of the
Secondary NDI results from interference with the normal urine protein/creatinine (P:C) ratio, and abdominal ultraso-
interaction of AVP and renal tubular AVP receptors, prob- nography. Ideally, all realistic causes of secondary acquired
lems with the generation of intracellular cyclic adenosine NDI should be ruled out before tests for primary pituitary
monophosphate (cAMP), problems with renal tubular cell and NDI and psychogenic PD are performed.
function, or loss of the renal medullary interstitial concen- Critical evaluation of urine specific gravity measured
tration gradient. Primary polydipsic disorders occur in dogs from several urine samples obtained by the client at different

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