Hypothyroidism

Hypothyroidism is the most common hormone imbalance of the dog.  It seems like it would be a straight forward problem: the body does not produce enough thyroid hormone, problems result, a test shows thyroid hormone level is low, the hormone replaced given in pill form, problem solved. Unfortunately, it is not quite so straight-forward in real life.  This article attempts to review the issues relevant to this condition and the pitfalls that keep it from being a simple problem.

WHAT IS THE THYROID GLAND AND WHAT DOES IT DO?

The thyroid gland is an "H" in one's throat. It produces two forms of thyroid hormone: "T3" which is the active form of the hormone, and "T4" which is the inactive form created to circulate in the bloodstream.  Production of T4 is regulated by the pituitary gland at the base of one's brain. (This gland is called the "master gland" as it regulates hormone production in the adrenal system, the thyroid system, the reproductive system and more). The pituitary produces a substance called "TSH" which stands for "thyroid stimulating hormone." When T4 levels are dropping, the pituitary gland stimulates the thyroid gland to make and release more T4.

Active thyroid hormone serves as a sort of a volume dial for metabolism. Since virtually every cell in the body can be affected by reduced levels of thyroid hormone it is not surprising that reduced levels of thyroid hormone lead to symptoms in multiple body systems.

WHAT IS HYPOTHYROIDISM?

In short, hypothyroidism is the natural deficiency of thyroid hormone. This deficiency is produced by immune-mediated destruction of the thyroid gland, by natural atrophy of the gland, by dietary iodine deficiency, or as a congenital problem.  In the dog, the first two causes listed account for almost all cases.

Hypothyroidim generally develops in middle aged or elderly dogs. Breeds with definite predisposition to develop hypothyroidism include: the Doberman pinscher, the Golden retriever, the Irish Setter, the Great Dane, the Dachshund, and the Boxer.

MANIFESTATIONS OF HYPOTHYROIDISM

Hypothyroidism is a classical disease with a classical collection of clinical signs.  One particularly well published survey of 162 confirmed hypothyroid dogs showed the following common findings:

• 88% had some kind of skin abnormality
   
• 40% had hair loss (often this starts on the tail leading to a "rat tail" appearance or a bald area around the collar is created)
  
  
  
• 22% had skin infection (often dogs are scaly and smelly due to an excessively oily coat)
   
• 14% had brittle or dry coats (often the outer hairs break off leaving a short, softer under coat, classically described as a "puppy-like coat")
   
• 49% were obese
   
• 48% were described as lethargic or listless at home
   
• 36% were anemic (had a reduced number of red blood cells  due to slowed red blood cell production in the bone marrow.)

One classical finding in hypothyroid dogs is a thickening of some tissues, especially of the face and head. The skin in particular thickens leading to more skin folds and what is classically referred to as a "tragic face." This thickening is called "myxedema" and can occur in some other tissues as well (such as facial nerves see later).

Cardiovascular Signs Hypothyroidism interferes with the electrical fibers that more or less provide the wiring for the heart. The rhythmic contractions of heart muscle as normally stimulated by these electrochemical fibers.  Abnormal rhythms or slow heart rate occur in as many as 26% of hypothyroid dogs; still, the significance of this in terms of overall lifestyle is not clear.  Many specialists feel thyroid supplementation should be started at a lower dose for patients with obvious heart disease.

Neurologic Signs According to surveys of confirmed hypothyroid dogs, only about 2-4% have nerve problems. There are several syndromes reported.

• "Polyneuropathy" In hypothyroidism, nerves simply do not conduct electrical impulses normally. This may account for some of the general weakness and listlessness seen in hypothyroidism.  Response to thyroid hormone therapy is rapid (improvement within the first week of treatment).
   
• "Focal Neuropathy" Single nerves can get entrapped as they exit the skull or spinal cord as they (like other tissues) swell with myxedema. Pressure on these nerves can lead to paralysis of the facial muscles and/or head tilt, bizarre eye motions and balance disruption ("vestibular disease").
   
• Central Nervous System- may represent abnormal electrical conduction within nerves; however, vascular disease has been found in hypothyroid dogs with central nervous system signs.  Clinical signs have involved ataxia (drunken gait), hemiparesis (weakness in front and back legs on the same side of the body), hypermetria (inappropriate measurement of steps), head tilt, circling & cranial nerve abnormalities. It may take several months of therapy to see a response. Coma from myxedema in the brain's tissues is a rare possibility but has been known to occur.

Ocular Signs Ocular changes are not common in hypothyroidism but the high levels of blood cholesterol and circulating fat can sometimes lead to eye changes. When these changes are seen, often thyroid testing is recommended.  Corneal dystrophy, an abnormal change in the clear covering of the eye, is such an eye sign. This finding is usually represented as a small white spot (sometimes a white circle) on the eye surface. At this degree it is only a cosmetic problem and does not interfere with vision. In more severe forms, painful bubbles can erupt on the corneal surface leading to ulceration. Obviously, this form would require treatment.

Conditions not proven (but previously suspected) to be associated with hypothyroidism: megaesophagus, laryngeal paralysis, infertility, and behavioral aggression.

TESTING FOR HYPOTHYROIDISM

One would think testing for hypothyroidism would be simple: a blood test of the T3 or T4 level could be checked and if it is low, the patient is hypothyroid.  Unfortunately, the situation is rarely so simple.

Testing with a Trial of Medication Sometimes the only way to test for hypothyroidism is to simply administer the medication for several months and see what happens.  Often an improvement in attitude and energy level is seen within the first week.  Hair re-growth takes substantially longer (typically 4 months minimum) as the follicles must "reawaken" and then grow a hair long enough to create a visible coat change.

TREATMENT OF HYPOTHYROIDISM

At least treatment of hypothyroidism is relatively straight-forward. Hypothyroidism is treated with oral administration of thyroid hormone (T4). Even hypothyroid dogs are perfectly capable of converting T4 to T3. Pills are given usually twice daily to start but may be dropped to once  a day after good thyroid control has been achieved.

There are many brands of thyroid supplementation available and prices are somewhat variable depending on the manufacturer.

TREATMENT IS FOR THE LIFE OF THE DOG.

Occasionally we are asked if it is reasonable to use dried or powdered thyroid glands of hogs or cattle as a more "natural" form of treatment. The answer is simply "no."  These products are not produced with adequate quality control to insure that they contain a reliable amount of thyroid hormone. Each dose may be completely different when such a product is used.

Re-testing later on?
Whenever an animal goes on a medication long term, periodic blood testing is a good idea. In the case of hypothyroidism treatment, it is important to know if the medication dose is too low or too high. Thyroxine (T4) is a very safe medication but if it is not given in adequate doses, obviously the patient will not be adequately treated. If the dose is too high and given for too long a time, excessive water consumption, weight loss, and restlessness can result.