As a veterinarian for more than 50 years and still actively involved in our profession, I feel compelled to write about two legends that persist among us regarding thyroid disorders and vaccines. I deal with these misunderstandings every week, not only in communicating with colleagues but also from their frustrated clients. Thyroid Disorders We all realize that thyroid dysfunction disorders are the most commonly seen endocrine condition of dogs and the second most common of cats, after diabetes. We also know that dogs typically get hypothyroidism, that much of it is caused by heritable autoimmune thyroiditis and that older cats suffer from hyperthyroidism. As the thyroid gland regulates metabolism of all body cellular functions, reduced thyroid function can produce a wide range of clinical manifestations, many mimicking those of other causes. Thus, recognition of the condition and interpretation of thyroid function tests can be problematic. MYTH Laboratory reference ranges accompanying results apply to all animals of that species. TRUTH All animals are not the same. Puppies and kittens have higher basal thyroid levels than adults. Geriatrics have lower basal thyroid levels than adults. Large/giant breed dogs have lower basal thyroid levels. Sighthounds have much lower basal thyroid levels. MYTH Diet and supplements given do not influence thyroid test results. TRUTH Feline hyperthyroidism can be induced by feeding commercial pet foods, treats and snacks containing excessive amounts of iodine. Excess iodine in dogs can induce autoimmune thyroiditis, as can eating glutens. Iatrogenic hyperthyroidism occurs when animals are fed the gullet or throat portion of raw red meat. Levels normalize upon removing this meat in four to six weeks. Other foods that impact thyroid function include soy, zinc, selenium, vitamins B6, D and E, and linoleic acid. MYTH Assessing thyroid function is usually inaccurate when certain drugs are given, or during sex hormonal cycles. TRUTH Basal thyroid levels are slightly lowered by phenobarbital, sulfonamides, steroids and excessive iodine. This factor should just be taken into account. Basal levels are lowered by estrogens (estrus, DES) and raised by progesterone (pregnancy, pseudopregnancy). MYTH Dogs should look clinically hypothyroid to warrant thyroid diagnostic testing. TRUTH Not necessarily. Classical clinical signs occur only after 70 percent or more of thyroid tissue has been destroyed or damaged. Other clinical and behavioral changes can present during the early phase. MYTH Behavioral changes are not associated with thyroid dysfunction. TRUTH Aberrant behavior including aggressiveness, phobias, hyperactivity, poor concentration, and seizures is associated with canine hypothyroidism and feline hyperthyroidism. MYTH Initial screening of thyroid function just needs a total T4. TRUTH T4 alone can give misleading results. It can overdiagnose hypothyroidism in nonthyroidal illness (NTI) or use of certain drugs; underdiagnose hyperthyroidism in cats or from thyroxine overdosage; inaccurately assess adequacy of thyroxine therapy; and fails to detect autoimmune thyroiditis. MYTH Accurate testing for free T4 requires the equilibrium dialysis (ED) method. TRUTH Some still favor the ED method, as earlier analog methods were less accurate. New technology offers accurate methodology, and is faster and less costly. MYTH Screening for thyroid function should always test thyroid stimulating hormone (TSH). TRUTH Unlike human thyroid function, TSH is only about 70 percent predictive because dogs have a more active alternate thyroid regulatory pathway through growth hormone. This assay requires a species-specific reagent (cTSH), as human TSH assays do not work in dogs (or cats). There is no feline TSH assay, and so cTSH is used. MYTH Accurate thyroid screening in older cats requires just a total T4, or T4 and free T4. TRUTH Basal thyroid levels in older cats should be lower than in adults. Other illnesses often lower T4, masking hyperthyroidism. Minimum testing needed is T4, free T4 and cTSH. FT4 by ED is usually high in hyperthyroidism but can be high in IBD, renal and liver disease, and neoplasia. Hyperthyroid cats have very low cTSH, whereas high cTSH occurs in cats with naturally occurring hypothyroidism, and in cats with iatrogenic hypothyroidism, i.e., secondary to methimazole or radioiodine-131 treatment. MYTH Thyroid therapy can be effective in dogs if given just once daily. TRUTH Thyroxine treatment is best given twice daily. Achieves better steady state over 24 hours; the half-life of T4 is only 12 to 16 hours. Dosing once daily results in undesirable peaks and valleys. MYTH Thyroxine can be given with food or in the food bowl. TRUTH Thyroxine binds to calcium and soy, and so it should be given apart from meals (one hour before or three hours after), regardless of what the product label says. MYTH Testing dogs on thyroxine only requires total T4, and the timing of the blood sample is irrelevant. Blood samples should be drawn four to six hours post-pill for BID therapy. Minimum testing needed is T4 and free T4. A thyroid antibody profile is required for all thyroiditis cases. Stopping thyroxine to retest basal capacity needs six weeks or more. Thyroid support or Thytrophin products are inadequate alone to fully correct true hypothyroidism and cannot resolve thyroiditis. MYTH Dogs with autoimmune thyroiditis don’t need thyroxine until clinical signs appear. TRUTH Dogs with autoimmune thyroiditis should be treated with thyroxine twice daily, even if basal thyroid levels are still normal (i.e. in “compensative autoimmune thyroiditis”). Therapy causes negative feedback inhibition of pituitary TSH output, which stops stimulation of thyroid receptors and stops thyroid autoantibody production. Thyroxine therapy is needed lifelong to prevent further autoantibody production. MYTH If healthy or treated for thyroiditis, these dogs can be used for breeding stock. TRUTH Thyroiditis is a heritable trait. Affected dogs, even if asymptomatic, should not be used for breeding purposes. Screen relatives annually from puberty; females during anestrus. Consider for breeding, if negative, after age 3 and preferably after age 6. MYTH Dogs treated correctly with thyroxine and yet have low total T4 need a dose increase. TRUTH Dogs oversupplemented with thyroxine can still test with low total T4. Dose increases are not appropriate because the body has excreted thyroxine faster to avoid thyrotoxicosis. Vaccines Animals properly immunized against the clinically important viral diseases have sterilizing immunity that not only prevents clinical disease but also prevents infection, and only the presence of antibody can prevent infection. An animal with a positive antibody titer test against these viruses has sterilizing immunity and should be protected from infection. If that animal were vaccinated it would not respond with a significant increase in antibody titer but instead may develop a hypersensitivity to vaccine components (e.g. fetal bovine serum, adjuvants). One should avoid vaccinating animals that are already protected. Furthermore, protection as indicated by a positive titer result does not suddenly drop off unless an animal develops a serious medical problem such as cancer or receives high or prolonged doses of immunosuppressive drugs. Viral vaccines prompt an immune response that lasts much longer than that elicited by other microbes and by classic antigen exposures. Lack of distinction between the two kinds of responses may be why practitioners think titers can suddenly disappear. But not all vaccines produce sterilizing immunity. Those that do include distemper virus, adenovirus, and parvovirus in the dog, and panleukopenia virus in the cat. Examples of vaccines that produce nonsterile immunity would be leptospirosis, bordetella, canine influenza, rabies virus, and the herpesvirus and calicivirus upper respiratory viruses of cats. While nonsterile immunity may not protect the animal from infection, it should keep the infection from progressing to severe clinical disease. MYTH There is little to no risk of vaccinating animals that are already immune. TRUTH Vaccines contain material designed to challenge the immune system of the pet, and so can cause adverse reactions. They should not be given needlessly and should be tailored to the pet’s individual needs. MYTH The initial series of puppy core vaccines are always safe and not immunosuppressive. TRUTH Immune suppression from MLV canine distemper, parvovirus and hepatitis vaccines coincides with the time of vaccine-induced viremia, from three to 10 days after vaccination. MYTH Half-dose vaccines cannot adequately immunize small toy dogs. TRUTH Some experts advocate the whole amount, as it provides the minimum immunizing dose. Our recent research giving half-dose of distemper and parvovirus booster to adult toy dogs elicited sustained protective immunity. Rabies vaccines must always be given in full dose, as required by law. MYTH Anesthetized animals can safely be vaccinated. TRUTH Vaccinating anesthetized animals is ill-advised, as a hypersensitivity reaction with vomiting and aspiration could occur. Anesthetic agents can have an immune modulating effect. MYTH Pregnant pets can safely be vaccinated. TRUTH Absolutely not. Small animal vaccine labels state not to vaccinate pregnant pets. MYTH Pets with diseases such as cancer or autoimmune diseases, or adverse vaccine reactions/hypersensitivity can safely receive booster vaccinations. TRUTH MLV products should be avoided as the vaccine virus may cause disease. Vaccination with killed, inactivated products may aggravate immune-mediated disease or be ineffective. For rabies boosters, local authorities may accept titers or a written exemption instead. MYTH Vaccines can be given less than two weeks apart if a different vaccine is being given. TRUTH The safest and most effective interval to immunize is three to four weeks apart. MYTH Puppy and kitten vaccine series should start early and continue until 16 weeks of age. TRUTH The last dose of vaccine given at 14 to 16 weeks old in dogs and 12 to 14 weeks in cats should immunize them. The rabies vaccine is safest given separately at least two weeks later (e.g. 18-24 weeks). MYTH New parvovirus and distemper virus field mutants are not adequately protected by current MLV vaccines. Current vaccines provide protection from all known viral isolates. Current vaccines provide both short- and long-term protection from challenge. MYTH Intranasal or oral Bordetella vaccine can be given parenterally. TRUTH These mucosal vaccines can cause a severe local reaction and may be fatal. MYTH The killed parenteral Bordetella vaccine given intranasally or orally produces immunity. TRUTH It will not immunize the dog. MYTH MLV parenteral vaccines for cats can be used intra-nasally. TRUTH Never. Mucosal (e.g. conjunctival/nasal) contact with these vaccines can cause disease. MYTH Disinfectant should be used at the vaccine injection site. TRUTH Disinfectants could inactivate a MLV product. MYTH If an animal gets only the first dose of a two-dose vaccine, the series must start over again. TRUTH As long as the second dose is given within six weeks, it should provide immunity. MYTH Vaccinated animals will not be immunized for several weeks after vaccination. TRUTH This is dependent on the animal, the vaccine and the disease. Fastest immunity is provided by canine distemper vaccines, within 24 hours. Immunity to canine parvovirus and feline panleukopenia virus takes three to five days. MYTH Dogs that fail to develop immunity to vaccines just need to be revaccinated. TRUTH Some will respond, if capable to do so. Others never will, as they are genetic vaccine “non-responders.” This trait runs in certain breeds and families. Dr. Dodds is the founder of Hemopet, a Garden Grove, Calif., animal blood bank, greyhound rescue and veterinary diagnostic laboratory. She is a proponent of minimum vaccine protocols. Originally published in the June 2016 issue of Veterinary Practice News. Did you enjoy this article? Then subscribe today!
