Health Issues Hypothyroid Article

GENETIC HEALTH – Hypothyroid CISC Winter issue 2005
By Sharon of Belglen Kennel

Hypothyroid - underproduction of thyroid hormones - is the most common endocrine problem in dogs. A very large percent of dogs with hypothyroid have autoimmume thyroiditis (also called lymphocytic thyroiditis) which is an heritable condition.

Actual clinical problems generally occur between 4-8 years of age in medium to large sized dogs. Both males and females are equally affected although some studies show spayed females may be more prone to it than intact females (one symptom females but not males might display is to reproductive function). The clinical signs of many diseases can appear like hypothyroid and so it is important to diagnose and treat other disease conditions first.

As an example, thyroxin (T4) is produced by the thyroid gland, as is T3. ‘Free T4’ and ‘Free T3’ are the actual, available hormones the body can use. The body also changes some T4 to T3 then the T3 is used by the body in the pituitary gland, liver, and kidneys. Other illnesses may affect this T4 to T3 process leading to insufficient T3 even though the thyroid gland itself is still functioning correctly. In this case the dog is not ‘hypothyroid’, adding thyroid medication will not help but treatment of the real underlying disease can also lead to ‘curing’ the secondary thyroid problem.

Diagnosis of a thyroid dysfunction in people is much simpler than in dogs. In dogs, an accurate diagnosis requires measurement and interpretation of at least three and possibly six different blood factors (the recommended tests are still in some dispute). Thorough genetic screening requires one more yet. For breeding stock in breeds with the possibility of hypothyroid occurring, the recommendation is to do a baseline screen after sexual maturity (over 1 year in males and 12-16 weeks after start of the first estrus cycle in females so that conflicting hormones are at their lowest point in the heat cycle). This should be repeated annually at least to 6 years of age then every second year. OFA now provides a dated thyroid registry for a one-time $15US fee.

Correct interpretation of results is very important. Any one test by itself is not conclusive in dogs (some individual veterinarians still use just one for diagnosis which can be inaccurate). T4 levels are normally lower in some breeds of dogs. They will also be lowered by other drugs, recent vaccination, heat cycle, or even the time of day. T3 fluctuates widely and can be within normal range even in hypothyroid dogs. Free T4 is a more true measure but the one accurate test for dogs currently available (equilibrium dialysis – OFA is requiring this method) is expensive and many labs use a different less accurate method.

Measurement of Thyroid Stimulating Hormone(TSH) has been used in people but dogs are less straightforward and both false negatives and false positives can be obtained. So the veterinarian needs to look carefully at all the results together to make an informed diagnosis. At least three indications in a complete Thyroid Panel are considered indicative of hypothyroid. According to Dr. Jean Dodds a full thyroid panel includes T3, T4, FreeT3, FreeT4, T3 Auto Antibodies and T4 Auto antibodies ($57.50US with her interpretation and $25US for adding TgAA). The presence of both T3AA and T4AA indicates autoimmune thyroiditis.


Autoimmune thyroiditis is also detected by the presence of Thyroglobulin autoantibodies (TgAA) in the blood and one position (now more accepted) is that breeding stock should be screened through this test as well as a full panel. Dr. Dodds believes that the presence of TgAA in the blood is the earliest indicator of future issues with two screens a year apart showing an increase in TgAA as absolute providing the other thyroid tests agree. Again careful timing is important as besides the stage of estrus, recent vaccinations can give a false positive. OFA’s registry uses the 6 TgAA as well as measurement of canineTSH and assessment of Free T4 through what they call a ‘gold standard technique’.

The form requests vaccination history but only recommends that a female not be in heat. OFA-approved laboratories must be used and annual retests have only veterinary (assess dog and fill out lab form and OFA form, collect blood, centrifuge to obtain 2 ml serum, ship overnight to an approved lab) and laboratory charges (University of Guelph is only Canadian approved lab and their fee is $55). Early detection can improve breeding choices and improve quality of life for the dog by preventing onset of advanced symptoms through earlier treatment.


Some typical symptoms of advanced hypothyroid include lethargy or mental dullness, weight gain, dry skin but not itchy, hair loss on trunk or tail and skin thickening on these areas, seeking out warm places to lie down, chronic skin or ear infections, and slow heart rate. However dogs may be hypothyroid without any of these symptoms but instead present with depression or unprovoked aggression, seizures, head tilt or other apparently nerve related conditions. Any of these problems should include thyroid function tests as a first or second step in the search for root causes. Thyroid medication should never be given without clear indicators of need through a Thyroid Panel. Blood levels need to be tested in 6-8 weeks for dose adjustment which is nearly always indicated.


Continued next issue.
References
www. upei.ca/~cidd/intro.htm
www. itsfortheanimals.com/HEMOPET.HTM
www.ofa.org
www.pubmedcentral.nih.gov