Hypothyroidism/Disorders of the thyroid gland

[k9cushings]

Some Cushing's dogs may also be affected with Hypothyroidism so testing for hypothyroidism is often part of the diagnostic process we go through with our Cushing's dogs. A full thyroid panel can be run from a single blood sample. Two of the tests that are usually included in a thyroid panel to check thyroid function are Canine TSH and Free T4 by equilibrium dialysis

Here is an excellent website about Canine Hypothyroidism which may affect our pets:

http://www.lbah.com/canine/hypot4.htm

Sometimes a Cushing's dog may be truly hypothyroid and need to be taking medication for that condition along with the Cushing's medication they are prescribed. But there is also a form of hypothyroidism called "Sick Euthyroid Syndrome" or "non-thyroidal illness" which may actually only be secondary to (caused by) another condition such as Cushings, and this type of hypothyroid condition will often resolve without need for medication once the excessively high cortisol production caused by the Cushing's is lowered and brought under good control.

from the above website:

The first scenario is called the sick thyroid syndrome or nonthyroidal illness (NTI).

In this situation the thyroid gland is normal, but there are factors that are suppressing it from secreting a normal amount of thyroxine into the bloodstream. These factors include medications like cortisone, valium, anticonvulsants, and sulfa antimicrobials. Diseases like Cushing's disease, diabetes mellitus, chronic renal failure, liver disease, and Addison's disease can also cause NTI.

When these factors are corrected, or these diseases are treated, the apparent hypothyroid problem corrects itself. No treatment with supplemental thyroxine is needed.

In the second scenario the thyroid gland is having a problem secreting adequate thyroxine due to one of the causes previously mentioned in the causes section. This is the hypothyroidism we need to treat with supplemental thyroxine.

How do we differentiate between a true hypothyroidism from the sick thyroid syndrome?

We have another blood sample that aids us, called the free T4 test by equilibrium dialysis. If this is low, and the signalment, history, and physical exam are consistent with this disease, then a diagnosis of hypothyroidism is made.

A knowledgeable Vet should know how to tell the difference between true Hypothyroid condition that requires medication and a case where the dog only has a "Sick Euthyroid" or "non-thyroidal illness" which does not require medication.

Here is another good website about thyroid function:

http://www.newmanveterinary.com/ThyroidBasics.html

and here's a link to another page on that website about Hypothyroidism which has some really good photos (if you put your mouse arrow over any of the photos and hold the mouse arrow there the picture will stop moving and stay still to give a better view)

http://www.newmanveterinary.com/Hypothyroid.html

Some of the symptoms of Hypothyroidism are very similar to some of the symptoms of Cushing's, so it can get confusing ...

A knowledgeable Vet should be able to figure out if the dog has Cushing's and is also truly Hypothyroid (and therefore needs medications to treat both conditions) or if the pet has Cushing's and "Sick Euthyroid Syndrome" (which would resolve once the cortisol levels are well controlled and without needing specific thyroid meds), or if the pet has only Cushing's or only a True Hypothyroid Condition, each of which would have a separate treatment protocol.

[k9cushings]

Disorders of the Thyroid Gland
Thomas K. Graves, DVM, PhD, DACVIM
VCM 331, Spring 2003

ThyroidDisordersGraves.pdf

How is Canine Hypothyroidism Diagnosed????

In the past, the most reliable test for canine hypothyroidism was the TSH stimulation test. Exogenous TSH administered to a normal dog causes a consistent rise in the serum concentration of TT4, whereas dogs with hypothyroidism exhibit little or no response to TSH. While there were plenty of gray areas in interpreting this test, it was a very useful diagnostic tool. Unfortunately, medical grade TSH is no longer available commercially, so the test is no longer done. Because of the weaknesses inherent in the available tests, it is probably best to use a combination of tests to confirm a diagnosis of canine hypothyroidism.

My personal opinion is that in order to make a diagnosis of hypothyroidism, a dog must have appropriate clinical signs and a low free T4, as well as at least two of the three following results: high TSH, positive anti-thyroglobulin antibodies, hypercholesterolemia. Other clinicians have less stringent diagnostic criteria. Thyroid biopsy would probably be the best test, but it is invasive and not practical.

Treatment:
Hypothyroidism is rewarding to treat. Treatment is simple and inexpensive – synthetic T4 supplementation (0.1 mg/10 pounds BID). Dermatologic signs usually being to improve within 4-6 weeks. Therapy is monitored by assessing clinical signs and by measuring serum T4 concentrations 4 – 8 hours after dosage administration. The prognosis, given appropriate treatment, is excellent, and most dogs treated for hypothyroidism live normal lifespans.

_______________________________________________

Canine Hypothyroidism: Fact or Fiction
Thomas K. Graves, DVM, PhD, DACVIM
Chief of Small Animal Medicine
Department of Veterinary Clinical Medicine
College of Veterinary Medicine
University of Illinois at Urbana-Champaign
Urbana, IL 61802

Canine Hypothyroidism Fact or Fiction Graves.pdf

Hypothyroidism: Myth vs. reality (Proceedings)
Apr 1, 2009
By: Thomas K. Graves, DVM, PhD, DACVIM
CVC PROCEEDINGS
http://veterinarycalendar.dvm360.com...tegoryId=45685

Authors almost always refer to hypothyroidism as a common (if not the most common) endocrine disease in dogs. but there is little evidence to support this claim. In fact, canine hypothyroidism may be one of the most over-diagnosed diseases in small animal medicine. Just about any medical disorder can be associated with decreases in circulating concentrations of thyroid hormone, causing hypothyroidism to be over-diagnosed. In addition, it is common practice for veterinarians to treat hypothyroidism empirically following a presumptive diagnosis. This practice is not recommended for several reasons. First of all, it is not scientifically sound medical practice. Secondly, it may allow another disorder to progress undiagnosed. Thirdly, unnecessary supplementation with thyroid hormone disrupts the normal regulation of TRH, TSH, and thyroid hormone production, the consequences of which are not understood.

Clinical signs of hypothyroidism result from generalized decreases in metabolic functions supported by thyroid hormone action. The clinical signs of hypothyroidism are multi-systemic and varied. Their onset is gradual. The classic presentation of a hypothyroid dog includes obesity, inactivity, mental dullness, decreased appetite, dermatological abnormalities, and intolerance to exercise and cold. These signs may or may not be present in all dogs with hypothyroidism.