Hypothyroidism and Diabetes


What is Hypothyroidism?
Hypothyroidism is the most common endocrine disorder in dogs.  Dogs diagnosed with hypothyroidism are usually between 4 and 10 years old. Since both hypothyroidism and diabetes is common in older dogs, it is not unusual to have both a diabetic and hypothyroid dog. Hypothyroidism is rare in cats and is usually the result of a thyroidectomy performed to treat hyperthyroidism. The information presented here is for dogs.

Hypothyroidism occurs when the thyroid gland, a small double-lobed gland in the neck, produces abnormally low levels of thyroid hormones.  The thyroid gland produces two hormones, thyroxine (T4) and triiodithyronine (T3).  

Hypothyroidism is most commonly due to destruction of the thyroid gland either from an immune-mediated process where lymphocytes of the immune system cause inflammation of the thyroid (lymphocytic thyroiditis), or atrophy.  

Less commonly, hypothyroidism can be due to cancer of the thyroid gland. It can also be due to dysfunction of the pituitary gland.  The pituitary gland secretes thyroid-stimulating hormone (TSH) which in turn causes the thyroid to produce thyroid hormone. Other illnesses, glucocorticoids, and malnutrition can also cause decreased TSH levels, which would lead to hypothyroidism.  

Congenital hypothyroidism causes dwarfism.  This is because thyroid hormone is necessary for normal development of the skeletal and central nervous systems.  

Signs develop slowly, effect several body systems, and can vary depending on the individual.  
  • General changes:
    • mental dullness
    • lethargy
    • exercise intolerance or unwillingness to exercise
    • tendency to gain weight without a corresponding increase in food intake
  • Changes in skin and hair coat
    • symmetrical hair loss beginning with the tail ("rat tail") and spreading over the rest of the body
    • dry, dull coat
    • easily epilated hair (hair is easy to pull out)
    • slow regrowth of hair
    • change in coat color
    • oily skin (seborrhea), thick, cool, puffy skin
    • poor wound healing and easy bruising
  • Changes in the reproductive system
    • infertility
    • shortened estrus in females, 
    • lack of libido, low sperm count, testicular atrophy in males
  • Changes in other systems:
    • slow heart rate, constipation, regurgitation

The physical signs of hypothyroidism can be caused by many diseases, so a complete history, physical findings, and blood tests are needed.  There are several blood tests available, but some are very expensive, not readily available, or are inconclusive.  Decreased T3 and T4 levels can also be caused by other diseases, so a thorough examination is important.  Typically, serum cholesterol and thyroid hormone concentrations are measured.  Since about 65-75% of hypothyroid dogs also have high serum cholesterol levels, low serum thyroid hormone levels in combination with increased blood cholesterol levels and appropriate clinical signs supports a diagnosis of hypothyroidism.

Hypothyroidism is treated by giving a pill containing synthetic thyroid hormone - Levothyroxine Sodium.  The drug is available under the trade names Soloxine, Thyro-Form, and Synthroid.  There are differences in how readily the different brands are absorbed, so it is recommended to use one product and not change brands indiscriminately. 

Response to thyroid hormone is gradual, with some signs such as activity and attitude improving as soon as 7-10 days after treatment begins.  Other signs, such as improvement in skin and coat condition may take 6-8 weeks.  Thyroid hormone levels are reevaluated periodically by a blood test and the dose may need to be adjusted. 

Considerations for Diabetics
Since both hypothyroidism and diabetes are common in dogs it is often necessary to deal with both conditions.

Diabetics should be started on a lower than normal dose of thyroid hormone.  Thyroid hormone therapy causes dramatic changes in the metabolic rate, therefore insulin requirements may change and the diabetes must be monitored closely.  Many owners report having to decrease their dog's insulin dose after thyroid hormone therapy was started.  Dramatic changes in the metabolic rate and metabolic processes occur with thyroid hormone therapy, so insulin therapy often needs to be adjusted.  Some owners have noticed that as the hypothyroidism is brought under control, the need for insulin decreases. One possible explanation for this is that hypothyroidism in dogs causes insulin resistance.  When the synthetic thyroid hormone is given and the thyroid levels are nearer normal, the insulin resistance begins to resolve and thus the higher insulin dosages which were needed before treatment are no longer required.

Signs of thyroid hormone overdose are similar to uncontrolled diabetes and include:

  • polyuria 
  • polydipsia
  • nervousness
  • weight loss
  • diarrhea
  • increased appetite
  • panting
  • weakness and fatigue
  • seeking cool areas
  • restlessness
  • increased heart rate, and fever.  
  • Hypoglycemia can also occur.  

Personal Experiences
Although each individual is unique, these experiences may help you understand how others have dealt with diabetic and hypothyroid dogs.

  • Udi Udi is both diabetic and hypothyroid, and close monitoring of his BG levels as I increased or decreased his soloxine dosages showed an increased need for insulin when his soloxine dosage was lowered, and decreased need for insulin as his dosage was increased. 


  • Kiri is diabetic, hypothyroid, and has Cushing's syndrome. You can read how Marilyn manages Kiri's three diseases.  



  • Pocket Companion to the Fourth Edition of Textbook of Veterinary Internal Medicine Stephen J. Ettinger.
  • The U.C. Davis Book of Dogs: A complete medical reference guide for dogs and puppies. Mordecai Siegal (Editor), et al.1995.
  • The 5 Minute Veterinary Consult: Canine and Feline. Larry Tilly and Francis W.K. Smith, Jr. 1997.
  • Veterinary Drug Handbook.  Second Edition.  Donald C. Plumb.  1995.


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vetmed Updated August 2001
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