Thyroid

An over abundance of thyroid hormone (called Hyperthyroidism) can lead to an overactive metabolism. Hyperthyroidism is not common, usually occurs in dogs with tumors of the thyroid gland. The T4 will be above normal and the TSH is below normal in this situation.A deficiency of thyroid hormone, ( called Hypothyroidism) can lead to a slow metabolism. Hypothyroidism is the most common hormone imbalance of the dog. The thyroid gland consists of two lobes located in front of the neck just above the larynx, near the trachea (windpipe). There are two forms of hypothyroidism in dogs. In one form, the immune system attacks the thyroid gland (autoimmune or lymphocytic thyroiditis) and a second form in which the gland seems to atrophy for other reasons. The most common cause of canine thyroid dysfunction is autoimmune thyroiditis (estimated 90% of cases), and tends to clinically manifest at 2 to 5 years of age. Decreased production of thyroid hormone is associated with destruction or degeneration of the thyroid gland.

The thyroid produces THYROID HORMONE. The two most important hormones are Tetraiodothyronine (thyroxin or T4) which is the inactive form created to circulate in the bloodstream and Triiodothyronine (T3), the active form of the hormone. When T4 is absorbed out the bloodstream and into tissue cells, it is converted into T3. Most of the circulating T4 is carried by blood proteins and is not available for tissue absorption; the portion that is not carried by proteins (the so-called “free T4”) is the portion that is able to enter tissues for activation. It’s primary function is the production of thyroxin, a hormone that allows the dog to maintain an adequate and consistent level of metabolism. 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.

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.

Hypothyroid dogs can be overweight, have neurological or muscular weakness in the hind legs, seizures, behavioral changes, anemia, skin and coat problems, cardiac problems, several types of digestive issues, urinary incontinence, infertility, and many other physical manifestations. Don’t surmise that your dog’s thyroid is normal because he or she is thin and has a good coat of fur.

Hypothryoidism can affect all organ systems including digestive, skeletal, muscular, respiratory, cardiovascular, nervous, skin, pituitary, adrenals, pituitary, renal, and the blood making components of the bone marrow. The thyroid influences protein synthesis, oxygen absorption, glucose absorption and many more biological functions.

Not enough thyroid hormone can cause a slow heart rate and therefore less perfusion of the kidneys and liver.

The immune system in a dog who is hypothyroid is not working to full capacity and it opens the door for many illnesses that may have been prevented with a properly functioning immune system. Dogs with degenerative neurological weakness of the hind end are often helped with thyroid medicine if blood tests indicate hypothyroidism.

These hormones are essential for life and have many effects on body metabolism, growth and development. The Thyroid gland is influenced by hormones produced by two other organs:

The Pituitary gland, located at the base of the skull produces thyroid stimulating hormone, (TSH).

The Hypothalamus, a small part of the brain above the pituitary, produces Thyrotropin releasing hormone (TRH).

This important little gland consists of two oval bodies located in either side of the neck. There are also tiny parts of thyroid tissue in other areas of the chest and even around the heart.

Symptoms of Hypothyroidism
**NOT all are required to be present for Hypothyroidism to be present**

lethargy
mental dullness
exercise intolerance
weight gain
cold intolerance
chronic infections including of the ear and bacterial skin infections
dry scaly skin and dandruff
coarse, dull coat
bilaterally symmetrical hair loss
chronic offensive skin odor
infertility of either sex
neuromuscular problems
cardiac abnormalities (bradycardia, cardiac arrhythmia’s, and cardiomyopathys)
gastrointestinal disorders
ocular diseases – corneal lipid deposits, corneal ulceration, uveitis, ‘dry’ eye, infections of eyelid gland
hair loss (often this starts on the tail leading to a “rat tail” appearance or a bald area around the collar is created)
Anemia (had a reduced number of red blood cells due to slowed red blood cell production in the bone marrow.)
Excessive skin pigmentation
Abnormal heat cycles – short or prolonged heat cycles, silent heats
False pregnancies
Seizures
Temperament changes
bleeding
low white blood cell count
low platelaet
stiffness
weakness
laryngeal paralysis
facial paralysis
knuckling or dragging of feet
muscle wasting
head tilt
drooping eyelids

DERMATOLOGIC DISEASES
• dry, scaly skin and dandruff
• coarse, dull coat
• bilaterally symmetrical hair loss
• “rat tail” or “puppy coat”
• hyperpigmentation
• seborrhea or greasy skin
• pyoderma or skin infections
• myxedema
• chronic offensive skin odor
CARDIAC ABNORMALITIES
• slow heart rate (bradycardia)
cardiac arrhythmias – Sinus bradycardia
Sinus bradycardia is a slow sinus mechanism (typically < 60 bpm in the dog; < 120 bpm in the cat). There may be concurrent sinus arrhythmia. This rhythm is associated with numerous clinical disorders, including: increased vagal efferent traffic, feline dilated cardiomyopathy, dysautonomia, neurocardiogenic syncope (e.g., in Doberman pinschers), CNS disease, endocrinopathies (hypothyroidism, Addison’s disease), hypothermia, and following administration of numerous drugs. Sinus bradycardia is common in dogs with primary sinus node dysfunction (and at risk for sick sinus syndrome). In most cases sinus bradycardia is self limiting, resolving as the primary condition is treated. When bradycardia is accompanied by clinical signs of hypotension, treatment with atropine (0.02 . 04 mg/kg), glycopyrrolate (0.01 mg/kg), and dopamine (2 10 ug/kg/min), with volume expansion, may be used. When related to paroxysmal syncope or near-death (asystole), ventricular antiarrhythmic drugs should be avoided to prevent suppression of escape activities.

• Sinus bradycardia: Increased vagal tone; elevated cerebrospinal fluid pressure; brain stem lesions; head trauma; gastrointestinal disease; neurocardiogenic syncope (e.g. in dilated cardiomyopathy of dogs); hypothermia; hypothyroidism; hyperkalemia; administration of beta-blockers, digitalis, or inhalation anesthetics; primary sinus node dysfunction in breeds at risk for sick sinus syndrome.
• Sinus tachycardia: Increased sympathetic tone; fear; excitement; pain; hypovolemia; hypotension; anemia; heart failure; hyperthyroidism; administration of vagolytic drugs, such as atropine, or sympathomimetics
Hypothyroidism and the Heart
Thyroid hormone is very important for normal cardiovascular function, so when not enough thyroid hormone is present neither the heart nor the blood vessels function normally. In hypothyroidism the heart muscle is weakened in both its contraction phase, and also its relaxation phase. This means that the heart cannot pump as vigorously as it should, and the amount of blood it ejects with each heart beat is reduced. In addition, because the heart muscle does not relax normally in between heart beats, a potentially serious condition called diastolic dysfunction may result. (Read about diastolic dysfunction). Furthermore, hypothyroidism reduces the amount of nitric oxide in the lining of the blood vessels, causing them to stiffen.

Cardiac symptoms of Hypothyroidism

Cardiac symptoms can be seen in anybody with hypothyroidism, but are especially likely in an individual who already has underlying heart disease. Common symptoms include:

* Shortness of breath on exertion and poor exercise tolerance. These symptoms, in most patients with hypothyroidism, are due to weakness in the skeletal muscles; but in patients with heart disease, the symptoms may be due to worsening heart failure.
* Slow heart rate (bradycardia.) The heart rate is modulated by thyroid hormone, so that in hypothyroidism the heart rate is typically 10 – 20 beats per minute slower than normal. Especially in patients who also have heart disease, however, hypothyroidism may worsen the tendency for premature beats (such as PVCs) and even tachycardias such as atrial fibrillation.
* Diastolic hypertension. One might think that, because a lack of thyroid hormone slows down the metabolism, animals with hypothyroidism might suffer from low blood pressure. Usually the opposite is true – the arteries are stiffer in hypothyroidism, which causes the diastolic blood pressure to rise.
* Worsening of heart failure, or the new onset of heart failure. Hypothyroidism can make well-controlled heart failure worsen, and can produce heart failure for the first time in patients with relatively mild underlying heart disease.
* Edema (swelling.) Swelling can occur as a result of worsening heart failure. In addition, hypothyroidism itself can produce a type of edema called myxedema, caused by an accumulation of abnormal proteins and other molecules in the interstitial fluid (fluid external to the body’s cells.)
* Worsening of coronary artery disease. While the reduction in thyroid hormone can actually make angina less frequent in patients who have angina, the increase in LDL cholesterol (bad cholesterol) and in C-reactive protein seen with hypothyroidism can accelerate any underlying coronary artery disease.

Hypothyroidism can be an extremely subtle condition, and often occurs without the typical, constellation of “textbook” symptoms doctors usually expect. It also occurs far more commonly than most doctors realize.

A new study in rats is giving researchers hope that more aggressive treatment of hypothyroidism and borderline hypothyroidism will result in a reduction of heart disease.

While further research is needed, results from a recent study entitled, “Low Thyroid Function Leads to Cardiac Atrophy with Chamber Dilation, Impaired Myocardial Blood Flow, Loss of Arterioles, and Severe Systolic Dysfunction,” suggest that low thyroid function has the potential to cause heart failure.

“We provided strong evidence that low thyroid function alone induced in rats eventually can cause heart failure,” said Dr. A Martin Gerdes, director of the Cardiovascular Research Institute and co-author of the study. “We also discovered that low thyroid function severely impaired cardiac blood flow due to a dramatic loss of the hearts small blood vessels (arterioles). Within six weeks after inducing low thyroid function in rats, half of the hearts small arterioles were gone.”

The effects of hypothyroidism were studied in rats on a short-term (6 weeks) and long-term (1 year) basis. What the research team learned is that hypothyroidism led to severe, progressive contractile dysfunction, chamber enlargement, and ventricular wall thinning despite a reduction in cardiac mass. Hypothyroidism induced in the rats also resulted in impaired myocardial blood flow due to a dramatic loss of arterioles. As a result, the team identified two new mechanisms by which low thyroid function may lead to heart failure.

The study is particularly significant because it was not clear previously if low thyroid function alone can actually cause heart failure or was just another risk factor.

In the absence of further research, however, Gerdes says some caution should be exercised since over treatment with thyroid hormones is known to cause adverse effects.
• cardiomyopathy
GASTROINTESTINAL DISORDERS
• constipation
• diarrhea
• vomiting
REPRODUCTIVE DISORDERS
• infertility or either sex
• lack of libido
• testicular atrophy
• hypospermia
• aspermia
• prolonged interestrus interval
• absence of heat cycles
• silent heat
• pseudopregnancy
• weak, dying or still born pups
OTHER ASSOCIATED DISORDERS
• IgA deficiency
• loss of smell (dysosmia)
• loss of taste
• glycosuria
• chronic, active hepatitis
• adrenal indocrinopathies
• pancreatic indocrinopathies
• parathyroid indocrinopathies

Above list compliments of W. Jean Dodds, D.V.M.

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). Clinical signs are slow to develop and are only evident when 75% of the thyroid gland is not functioning.

Why do skin problems seem to go hand and hand with thyroid imbalance? In order for dogs to have healthy skin and coats, the cells that make up the body must be healthy and multiply. It is not fully understood, but the thyroid gland is essential in regulating the metabolism of each individual cell. Whenever cells cannot function efficiently, once of the visible results is the skin will loose it’s elasticity, and can ultimately result in dry, crusty, scaly skin. The end result is a dog with a sparse coat, and a dull hardened gray skin that will have an obnoxious odor. A dog left untreated will be miserable with uncontrollable itching all over.

Not enough thyroid hormone can cause a slow heart rate and therefore less perfusion of the kidneys and liver.

Thyroid Panels and antibody tests should be used for genetics screening of apparently healthy animals to evaluate their fitness for breeding. Common tests that evaluate thyroid gland function by measuring thyroid hormone concentration in the blood are Total T4, TotalT3, Free T4 and TSH.
What is T3, T4, TSH, TgAA?

These are tests that evaluate the thyroid gland function by measuring the thyroid hormone concentration in the blood.
T3 and T4 levels are used as an indicator of thyroid functions.

cTSH (canine Thyroid Stimulating Hormone) Test, is the current “gold standard” test for measuring thyroid gland function. In this test, T4 concentration in the blood are measured BEFORE and 6 hours AFTER the administration of TSH.

TgAA (Thyroglobulin Autoantibodies) This test is an indication of the presence of the autoimmune process in the dog’s thyroid.

Your Vet may suggest to get a Total T3 and T4, plus Free T4 by equilibrium dialysis, Free T3, endogenous TSH (thyroid stimulating hormone) and TgAA (Thyroglobulin Autoantibodies).

The T4 and TSH and the FT4D assay are the best for diagnosing hypothyroidism, but do not identify the cause. The TgAA ( Thyroglobulin Autoantibody) assay is a test used that is highly specific for lymphocytic thyroiditis. Information from this three-part testing panel is used in the OFA thyroid registry database as the best possible means of diagnosing thyroid disease.

The first two tests tell us if the dog is hypothyroid and the third reveals if the dog has inherited thyroid disease. So many other things can give false low T3 and T4 levels: such as the time of day, physical condition of the dog, concurrent medication, and concurrent illness.

Other blood tests may be performed to evaluate other body systems since concurrent diseases such as Cushing’s disease (characterized by an abnormal production of steroids) or Diabetes can cause low levels of circulating thyroid hormone. However, controlling these disease processes usually eliminates the need for hypothyroidism treatment. Other systemic illnesses such as kidney and liver insufficiency, heart failure, or immune system disorders can falsely lower thyroid levels and this phenomena is known as euthyroid sick syndrome.

What causes Hypothyroidism?
The causes of Hypothyroidism are classified as Primary, Secondary or Tertiary.

Primary causes are the most common and are associated only with the thyroid gland.

Secondary causes are associated with the pituitary gland, such as a deficiency of TSH.

Tertiary causes, such as a deficiency of TRH, are associated with the Hypothalamus.

Dogs that are negative at 1 year of age may become positive at 6 years of age.
Exposure to drugs, many medications can adversely affect thyroid function.
Less than 5% of the cases of hypothyroidism are related to disease within the pituitary gland. In addition, hypothyroidism can be induced by prolonged administration of glucocorticoids (steroids).
Toxin and Chemical preservatives.
Nutritional imbalance or deficiencies.
Congenital Anomalies.
Hereditary.
Bacterial Infection caused by Strep, Staph or Pneumococcus.
Cumulative stress.

Other Points
The fact is that we don’t know the exact “normal” thyroid level for each breed. Different breeds have different “normal” thyroid levels.

Some evidence has been pointing towards the possibility of common environmental toxins, vaccine overload, and the diet being fed to today’s dogs being the causative agent of hypothyroidism.

Dogs need a lot more thyroid medication per lb. (or kg.) than people do so their dose is about ten times higher. Dogs also metabolize thyroid medication more quickly than people do (the half-life of thyroxine in dogs is 12-16 hours, whereas it is days in people). Therefore, they need to take their medication twice a day, rather than once a day- as humans do.