Page 926 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

CHAPTER 6

ENDOCRINE SYSTEM
VetBooks.ir Janice Kritchevsky and François-René Bertin 901

The endocrine system is comprised of the with overproduction or underproduction of
glands and tissues that release hormones into hormones, or with altered tissue responses to
the bloodstream, usually in response to a signal them. Tissue responses are not only mediated
from the hypothalamus or pituitary gland. via the classic, blood-borne hormones, but
Hormones are synthesised, stored and secreted often by local paracrine substances. Currently,
by the gland, and serve to regulate the function the interplay between hormonal and paracrine
of distant organs or tissues. Hormone secre- factors is the subject of intense research.
tion occurs in response to a specific stimulus
and is highly regulated, usually on several levels DIAGNOSTIC WORK-UP
including the hypothalamus, the end organs and
the gland itself. Hormones may have secondary History
effects that are not as tightly regulated, which A thorough history is an important part of the
at times may result in pathological states. For approach to cases in which an endocrine disorder is
example, insulin is released in response to blood suspected. The breed, age and primary complaint
glucose concentration in a highly regulated can often help a clinician target the affected sys-
manner, but the increased blood insulin needed tem. The clinician should focus on diet (e.g. type of
to maintain euglycaemia in a horse with insulin feed, quantity ingested), presence of other disorders
dysregulation results in alterations in fat and (e.g. sepsis, recurrent infections, chronic lameness
protein metabolism. Thus, the result of chronic or delayed healing) and possible concurrent treat-
hyperinsulinaemia can be decreased fat mobili- ments (e.g. non-steroidal anti-inflammatory drugs
sation and increased adiposity. [NSAIDs]). It is important to consider what time of
Transportation of hormones in the blood- the year the horse’s problems were first noticed, and
stream usually involves protein-binding, with if they wax and wane with the seasons. If possible,
only the free fractions of hormones being water consumption and frequency and appearance of
metabolically active. In order to regulate urination should be noted.
target tissues, an appropriate concentration of
hormone must be able to diffuse into the tissues Physical examination
and bind to a specific receptor either on the A complete physical examination is an essential part
target cell membrane or within the cell. Excess of the diagnostic work-up for all diseases. For com-
hormone must be metabolised into an inac- plaints that are likely to have an endocrine cause,
tive form and either be taken back up by the particular attention should be paid to body weight
gland for reprocessing and storage or be elimi- (BWT). In addition to noting whether an animal is
nated through the liver or the kidneys. Most underweight or overweight, it is important to evalu-
endocrine systems are tightly controlled by ate subcutaneous fat distribution and establish a
positive and negative feedback, with the target body condition score (BCS). Additionally, hair coat
organs releasing a product that inhibits the (e.g. length, condition, areas of alopecia) and men-
release of the stimulatory or releasing hormone. tal alertness and activity level (e.g. depression, leth-
Endocrine diseases usually involve problems argy, apprehension and exercise intolerance) should