This is the medical term for an underactive thyroid gland. One of the many things the thyroid gland is responsible for is to regulate metabolism. When not enough thyroid hormone is secreted, the metabolism slows;  secreting too much results in hyperthyroidism and a too-rapid metabolism. The thyroid gland is actually controlled by another gland, the pituitary gland. It signals the thyroid to produce its hormone and is responsible for the amount of thyroid hormone in the blood.
Over-medication of a hypothyroid condition can show up as hyperthyroid test results because the treatment is replacement therapy of the lacking thyroid hormone.
Symptoms of hypothyroid are dry skin and coat, often a loss of hair at the rear portion of the body, sluggishness and weight gain even if the animal has his/her meals reduced, skin that is cold to the touch. The animal may also deliberately seek out warm places to lie. These symptoms are shared with many other medical conditions; as a result, hypothyroidism isn't always a first thought when diagnosing. 
Dogs with hypothyroidism, Cushing's disease, or diabetes seem to tend to produce less than the normal amount of tears.  The continuing lack of enough lubrication in the eyes can lead to dry eye.  Other eye-related conditions due to hypothyroidism are also possible.
In many hypothyroid cases, the true cause of the lack of thyroid function is never discovered--it's referred to as idiopathic hypothyroidism. The other is known as lymphocytic thyroiditis, where the body begins producing antibodies against the thyroid gland. 
Symptoms of hypothyroidism may not develop until 75% of the thyroid gland is destroyed; it may take 1-3 years from development of the condition to this point. Idiopathic and lymphocytic thyroiditis causes account for over 95% of hypothyroidism in dogs. 
In less than 10% of hypothyroidism cases, the problem is not with the thyroid gland itself, but with the pituitary gland in the brain. The pituitary gland produces a thyroid stimulating hormone (TSH);  without this hormone to signal the thyroid gland to produce its thyroid hormone, the thyroid gland remains inactive. 
Lack of iodine in the pet's diet can result in insufficient production of thyroid hormones with the result being a type of hypothyroidism. 
Treatment is to replace the missing thyroid hormone,  which will need to be done for life. With proper continuing treatment, a normal life is expected. Intervet has introduced a new oral liquid replacement therapy called Leventa. Unlike tablet replacement treatments, which need to be given twice daily, Leventa is able to be given only once a day. 
Hypothyroidism and diabetesEdit
Dogs who also have diabetes have problems with regulation when their thyroid is underactive.  Treating the thyroid condition with replacement therapy--natural or synthetic thyroid hormone often resolves the diabetic regulation problem in the process. There's more than one test to determine thyroid function.  Just as various drugs are capable of influencing false results with urine glucose and urine ketone tests and to alter the blood glucose values of those with diabetes, some commonly-used drugs are able to do the same with thyroid tests and function.
Hypothyroidism can cause insulin resistance-meaning more insulin needed,  but it can also decrease the body's metabolic rate. If this is the case, the decrease in the metabolism would reflect as decreased insulin requirements.  It's reasonable to suspect hypothyroidism in addition to diabetes when there is continued hyperglycemia and weight gain. With only diabetes mellitus, the lack of blood glucose control would be expected to result in the opposite--weight loss. 
- ↑ Canine Hypothyroidism. Thyroid-info.com.
- ↑ Hypothyroidism-Vague, Nonspecific, but Serious. NAVC (2004).
- ↑ Canine Hypothyroidism. Southpaws.com.
- ↑ Williams DL, Pierce V, Mellor P, Heath MF. (2007). Reduced tear production in three canine endocrinopathies. Journal of Small Animal Practice.
- ↑ For the Dry Eyes in the House. NAVC (January 2003).
- ↑ Plummer, Caryn E., Specht, Andrew, Gelatt, Kirk N. (December 2007). Ocular Manifestations of Endocrine Disease. Compendium.
- ↑ Durkan, Samuel (January 2008). Endocrine Emergencies. DVM 360.
- ↑ Hypothyroidism: Defining Myxedema in Dogs. North American Veterinary Conference (2006).
- ↑ Atkinson, A., Auber, I. (2004). Myxedema Coma With Respiratory Depression-page 11. Canadian Veterinary Journal.
- ↑ Pullen, William H., Hess, Rebecka S. (2006). Hypothyroid Dogs Treated with Intravenous Levothyroxine. Journal of Veterinary Internal Medicine.
- ↑ Hypothyroidism. Long Beach Animal Hospital.
- ↑ 12.0 12.1 Leventa-Hypothyroidism Treatment. Intervet.
- ↑ Thyroid-Stimulating Hormone. College of Veterinary Medicine-Colorado State.
- ↑ Iodine Requirements in Dogs. Pet Education.
- ↑ Washabau, Robert J. (2009). Canine Pancreatic Disease: What's New in Diagnosis and Therapy?. WSAVA.
- ↑ Thyro-Tabs Information Leaflet-Page 22. Small Animal Endocrinology.
- ↑ Leventa.com. Intervet.
- ↑ Ford SL, Nelson RW, Feldman EC, Niwa D. (1993). Insulin Resistance in Three Dogs With Hypothyroid & Diabetes Mellitus. Journal of the American Veterinary Association.
- ↑ Canine Hypothyroidism. Dechra-US.
- ↑ Leventa Diagnostic Protocol-Canine Hypothyroidism. Intervet.
- ↑ Daminet, Sylvie, Ferguson, Duncan C. (2003). Influence of Drugs on Thyroid Function in Dogs. Journal of Veterinary Internal Medicine.
- ↑ Effect of Hypothyroidism on Insulin Sensitivity in Dogs-Page 29, Abstract #94. ACVIM (2008).
- ↑ Better Medicine E-Newsletter. Intervet (June 2006).
- ↑ Cook, Audrey (1 April 2010). Identifying the reasons behind difficult-to-control diabetes in dogs. DVM 360.
- ↑ Cirillo R, Balzano S, Cossu E, Bartalena L, Solinas MP, Falcone M, Balestrieri A, Martino E. (1988). The effect of altered thyroid function on serum fructosamine concentrations. Clinical Biochemistry.
- ↑ Hypothyroid and Elevated Fructosamine Tests. North American Veterinary Conference (2003).
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details hypothyroidism & hyperthyroidism
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