Autoimmune thyroiditis is the most common cause of primary hypothyroidism in dogs. The disease has variable onset, but tends to clinically manifest itself at 2 to 5 years of age. Dogs may be clinically normal for years, only to become hypothyroid at a later date. The markers for autoimmune thyroiditis, autoantibody formation (autoantibodies to thyroglobulin, T4 or T3), usually occur prior to the occurrence of clinical signs. The majority of dogs that develop autoantibodies have them by 3 to 4 years of age. Development of autoantibodies at any time in the dog's life is an indication that the dog probably has the genetic form of the disease. Using current technology only a small fraction of false positive tests occur.

As a result of the variable onset of the presence of autoantibodies, periodic testing is necessary. Dogs that are negative at 1 year of age may become positive at 6 years of age. Dogs should be tested every year or two to be certain they have not developed the condition. Since the majority of affected dogs will have autoantibodies by 4 years of age, annual testing for the first 4 years is recommended. After that, testing every other year should suffice. Unfortunately, a negative test at any one time will not guarantee that the dog will not develop thyroiditis.

This data can be used by breeders in determining which dogs are best for their breeding program. Knowing the status of the dog and the dog's lineage, breeders and genetic counselors can decide which breedings are most appropriate for reducing the incidence of autoimmune thyroiditis in the offspring. The Animal Health Diagnostic Laboratory at Michigan State University has the largest pooled database on breed prevalence of autoimmune thyroiditis (Table 8).

Table 8: Michigan State University thyroid statistics

October 2002

General procedures

Purposes
To identify those dogs that are phenotypically normal for breeding programs, and to gather data on the genetic disease of autoimmune thyroiditis.

Clearance issued
A breed registry number will be issued for all dogs found to be normal at 12 months of age or older. Age will be noted on the certificate since the classification can change as the dog ages. There is an initial OFA fee and no charge for recertification at a later date. It is recommended that reexamination occur at ages 2,3,4,6, and 8 years.

Preliminary evaluation
Dogs less than 12 months of age can be evaluated for the owner's information, however few dogs are positive at that age.

Dogs with autoimmune thyroiditis
All data, whether normal or abnormal, should be submitted to help assure accuracy of the database. There is no OFA fee for entering an abnormal evaluation of the thyroid into the data bank. Abnormal information will not be released into the public domain unless the owner gives permission for this release by initialing the appropriate line on the application form.

Thyroid abnormalities fall into several categories, and two types are defined by the registry:

1. Autoimmune Thyroiditis - Known or suspected to be heritable.
2. Idiopathic Hypothyroidism - Of unknown origin.

Dogs with laboratory results that are not definitive will be considered as equivocal. It is recommended that the test be repeated in three to six months.

Classification

Thyroid classifications are based on the most current and scientifically validated tests available.

Indices of thyroiditis
Free T4 by dialysis (FT4D)-This test is considered to be the "gold standard" for assessment of the thyroid's production and cellular availability of thyroxine. FT4D concentration is expected to be decreased in dogs with thyroid dysfunction due to autoimmune thyroiditis.

Canine thyroid simulating hormone (cTSH)-This test helps determine the site of the lesion in cases of hypothyroidism. In autoimmune thyroiditis, thyroid gland function is reduced, while the pituitary gland continues to function normally. Therefore, when FT4D levels fall due to a malfunctioning thyroid gland, the pituitary produces elevated levels of cTSH in an attempt to stimulate thyroid production. Thus, the cTSH concentration is expected to be abnormally elevated in dogs with thyroid atrophy from autoimmune thyroiditis.

Thyroglobulin Autoantibodies (TgAA)-This test measures the level of thyroid autoantibodies (antibodies directed against normal body tissue). Positive levels of thyroid antibodies are an indication that an autoimmune process is damaging the dog's thyroid gland.

1. Normal
   FT4D within normal range
   cTSH within normal range
   TgAA is negative.
2. Positive autoimmune thyroiditis
   FT4D less than normal range
   cTSH greater than normal range
   TgAA is positive
3. Positive compensative autoimmune thyroiditis
   FT4D within normal range
   cTSH greater than or equal to normal range
   TgAA positive
4. Idiopathically reduced thyroid function
   FT4 D less than normal range
   cTSH greater than normal range
   TgAA negative
5. All other results are considered equivocal

Laboratory certification

In an attempt to standardize testing methodology, laboratories are required to pass a certification process. Laboratories may apply, and if successful will be approved to perform analysis for OFA thyroid certification.

The following laboratories are approved, and should be contacted directly for the appropriate submission forms (other than the OFA application form), sample handling procedures, and laboratory service fee before collecting the sample.

Animal Health Diagnostic Laboratory
B629 W. Fee Hall-B
Michigan State University
Lansing, MI 48824-1315
517-353-0621

New York State Animal Health Diagnostic Laboratory
College of Veterinary Medicine, Cornell University
Upper Tower Rd.
Ithaca, NY 14852
607-253-3673

Animal Health Laboratory
University of Guelph
Bldg. 49, McIntosh Lane
Guelph, Ontario, Canada N1G 2W1
519-824-4120 ext. 4518

Veterinary Diagnostic Laboratory
College of Veterinary Medicine
University of Minnesota
1333 Gortner Ave
St. Paul, MN 55108
612-624-0761

University of California
Veterinary Medical Teaching Hospital
Clinical Pathology, Chemistry, Room 1017
1 Garrod Drive
Davis, CA 95616
530-752-7380

Texas Veterinary Medical Diagnostic Laboratory
1 Sippel Rd.
College Station, TX 77843
979-845-3414

Vita-Tech
1345 Denison St.
Markham, Ont., Canada L3R 5V2
800-667-3411
905-475-7309 fax

Instructions for testing

1. The veterinarian or owner must obtain the "Application for Thyroid Database" from the Orthopedic Foundation for Animals, Inc. (phone 573-442-0418), or online at www.offa.org.
2. The veterinarian and owner must complete their respective portions of the form.
3. Two milliliters (2 ml) of serum are needed for testing, and the serum sample must be from freshly collected blood. Use a plain "red-top" tube for blood collection. Do not use a serum separator tube with clot additives or any other type of plasma collection tube. After collection, place the blood sample in the refrigerator for 60 to 90 minutes to allow clotting. Centrifuge, collect the serum, and transfer to a plain plastic or glass tube suitable for shipping. Clearly label the sample with the owner's name, animal's identification, date of blood collection, and "OFA Thyroid Panel." If the specimen is to be stored for more than 12 hours prior to shipping, frozen storage is recommended.
4. Ship to the approved laboratory of choice via an overnight courier service. It is recommended that all specimens be packaged properly and shipped so they are received either chilled or frozen. Serum samples arriving unchilled or at room temperature within 48 hours of the collection date will be accepted. However, samples arriving after this time must be received either chilled or frozen in order to be accepted for registry testing. Contact the laboratory for further information as necessary.
5. Female dogs should not be tested during an estrus cycle. The date of last routine vaccination should be noted.
6. Please do not submit whole blood, clotted blood, or plasma.
7. Severely lipemic or hemolyzed specimens are also unacceptable.
8. Test results will be mailed or faxed only to the submitting veterinarian and the Orthopedic Foundation for Animals, Inc.. Results will not be available from the laboratory by telephone. The OFA will send a report to the veterinarian and to the owner.

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