Introduction
Hypothyroidism is one of the most common deficiency disorders among canines although it can not be easily diagnosed. It was first diagnosed in canines (dogs) in the 1970s (Volhard, & Volhard, 2011). It is mainly caused by an impairment of the thyroid gland responsible for the production and secretion of essential thyroid hormones (Bell, Latimer, LeRoy & Moore, 2005). This results in a significant decrease in the rate of metabolism. The disease is the most common endocrinopathy of the cat, dog, and other canines. Veterinary experts observe that the disorder is very common in canines (Mecklenburg, Linek & Tobin, 2009). The effects of hypothyroidism are so vast that they can spread through other body organs and hence altering their normal functioning. This explains why the disorder is associated with so many other medical problems. In dogs, the thyroid gland exists as two halves. The shape as well as the functions of the gland is similar to that in human beings.
Hypothyroidism in dogs may be classified into; primary hypothyroidism, secondary and tertiary hypothyroidism. The classification depends on where the cause lies; either in the thyroid gland, pituitary gland or in the hypothalamus (Flood & Hoover, 2009). The thyroid gland is situated in the trachea, located just below the larynx. The hormone released from this gland can either be active or inactive. The active hormone is referred to as triiodothyronine (T3) while the inactive form is commonly known as T4 (thyroxine) (PetWellbeing, 2011). Once the inactive hormone is released into the blood stream, it is soon converted into T3 and can then penetrate specific body tissues. The amount of T4 produced is normally regulated by the anterior pituitary gland. The research paper will analyze hypothyroidism in canines in terms of its causes, signs and symptoms, diagnosis and available treatment options. Due to the large number of species that can potentially be affected by this disorder, this paper will focus mainly on dog hypothyroidism.
Causes of Hypothyroidism
Common causes of hypothyroidism in dogs have been established and they are responsible for about 95% of all primary hypothyroidism (Day, 2008). Lymphocytic Thyroiditis and Idiopathic Atrophy are the two most known causes of hypothyroidism due to thyroid gland disorder. According to research findings, Lymphocytic Thyroiditis is a genetic predisposition to hypothyroidism and accounts for about 50% of all primary hypothyroidism cases (Mecklenburg et al., 2009; Day, 2008). It is believed that its cause may be associated with autoimmune polyglandular syndrome in some animals. This is a situation where the body system releases and circulates harmful antibodies that fight the thyroid gland. An impaired thyroid gland results in decreased production of thyroid glands. Lymphocytic thyroiditis is characterized by the infiltration of immune cells that automatically produces antibodies to the thyroid tissue. Inflammatory cells also infiltrate into the thyroid gland where they interact with hormones. It is this interaction that is responsible for the formation of immune complexes in the thyroid gland resulting in inflammation and series of biochemical reactions that in turn damage and degenerate the thyroid gland (Day, 2008). Studies estimate that about 42% to 59% of all dogs suffering from thyroiditis have antithyroglobulin antibodies (Mecklenburg et al., 2009). This dangerous process marks the onset of hypothyroidism. Hypothyroidism has been found to affect large and gigantic breeds of dogs which include the Great Dane, Douge de Bordeaux, English Mastiff, Leonberger and Golden Retriever (Coile, 2009).
Idiopathic Atrophy, on the other hand, is believed to mark the last ages of lymphocytic thyroiditis. It is characterized by sudden increase in the number of small thyroid follicles which are normally responsible for the synthesis of hormones (Ward, 2007). Due to their very small sizes, little hormones will be synthesized. Any attack on the thyroid gland causes low circulation of thyroid hormones and lack of feedback inhibition resulting in the formation of thyroid-stimulation hormone (TSH). The TSH then stimulates the gland triggering an increase in the number of the small follicles. This chain of reactions causes the thyroid tissue to fail in its functions.
Neoplasia is another cause of primary hypothyroidism although not very common. The Squamous Cell Carcinoma infiltrate into the thyroid gland and subsequently destroys the thyroid tissue. Thyroid Carcinoma also enters the same gland cause extensive damage. However, studies have shown that these tumors are not known to synthesize hormones (Coile, 2009). This implies that the destruction of the thyroid tissue and not hormones are responsible for the onset of hypothyroidism in animals.
Decreased secretion of normal (TSH) by the pituitary gland causes a special type of thyroid disorder known as secondary hypothyroidism. The result is the weakening and degeneration of the thyroid gland causing this category of hypothyroidism. However, this cause is responsible for less than 5% of hypothyroidism prevalence. The pituitary gland regulates a number of glands in the body including the adrenal and gonadal glands. Any problems with the pituitary will most likely result in several clinically significant signs associated with other abnormalities. Moreover, the pituitary gland is affected by any neurological deficiencies due to its nearness to the base of the brain (Ward, 2007). Generally, secondary hypothyroidism is caused by genetic malformation which is present at birth, malnutrition, use of certain drugs, and neoplastic invasion or destruction. There has been no scientific proof that tertiary hypothyroidism can occur in dogs. This has been occasioned by the lack of reliable assays that can effectively measure the circulating Thyroid-releasing hormone (TRH) (or TSH). Research into the development of the assays is ongoing. Proper diagnosis and understanding of the causes of hypothyroidism in canines are critical for successful treatment (Bonham, 2005).
Signs and Symptoms of Hypothyroidism
Given the role of thyroid hormones in virtually all organ systems of the body, any insufficiency will cause abnormalities in these organs (Volhard, & Volhard, 2011). This makes it easy to identify most of the signs and symptoms of thyroid disorder. However, the many signs and symptoms accompanying hypothyroidism makes it to be easily mistaken for many other diseases with similar manifestations like cancer and diabetes (Flood & Hoover, 2009). This calls for a very keen diagnosis of hypothyroidism disorder. Several signs and symptoms of hypothyroidism have been documented by veterinary specialists. An infected dog will show mental dullness or sudden changes in behavior, sluggishness, and increased appetite causing obesity or weight gain. Over 75% of dogs suffering from hypothyroidism have increased levels of blood cholesterol (Case, 2005). More signs include the dog’s inability to tolerate low temperatures, septic wounds due to poor healing, shaggy skin and hair coat, abnormality in hair turnover or loss of hair, the hair becomes brittle and dull, and pigment alteration. The sick dog will also have an oily or dry skin and the skin may be thickened due to a swelling as a result of abnormal dermal milieu turnover (myxedema) (Gross, 2005). Generally “sad” expression on the face, odiferous and itchy skin caused by secondary bacterial and yeast dermatitis, diarrhea, or constipation, and occasional bleeding are other common manifestations of hypothyroidism disorder.
The signs may also be neurological characterized by seizures and nervous system deficits. Problems in the nervous system may cause blindness, droopy eyelids, paralysis of eyelids and low production of tears resulting in the irritation, reddening, and drying of the eyes, staggering due to vestibular deficits, dragging of feet, general body weakness, and problems in swallowing on some occasions (Coile, 2009). These signs and symptoms are generally easily to identify but further physical examination and laboratory work will yield even more symptoms of hypothyroidism.
These cardiovascular signs can be arrhythmia and bradicardiya, which are abnormal electrical activity in the heart. Heart arrhythmia which is characterized by slower rate of heart beat, anemia, and high levels of serum cholesterol are some of the signs that may manifest upon further examination (Ward, 2007). There are also neurological symptoms that can come along with this disease as well. This may include such problems as those associated with the central nervous system, focal neuropathy or polyneuropathy. Focal neuropathy is caused when there is an increase in pressure on the skull or spinal cord. On the other hand, failure of the nerves to conduct normal nerve impulses in the body causes polyneuropathy. Furthermore, there are ocular signs such as increased blood pressure and cornea dystrophy, which causes the covering of the eye to become slightly bluish (Flood & Hoover, 2009). Although no fixed age marks the onset of hypothyroidism, research has shown that the disease occurs mostly in dogs between 4 and 10 years of age. All these signs and symptoms may coexist and this explains why the diagnosis of the disease can be quite difficult.
A specific case can be obtained from a study published by Flood and Hoover (2009) in the Canadian Veterinary Journal. They carried their investigation on an old male Alaskan malamute by monitoring it for two weeks. They found out that the eight-year-old malamute exhibited symptoms of lethargy, inappetence and had a swollen abdomen. After physical examinations were done, the dog was found to be obese and the heart sounds were muffled due to an arrhythmia (atrial fibrillation). Atrial fibrillations were confirmed by performing an electrocardiogram. The rapid and irregular heartbeats caused poor flow of blood to the rest of the body. This was found to occur when beats of the upper and the lower chambers of the heart were out of synch (Flood & Hoover, 2009). Moreover, the dog had facial myxedema usually caused by the accumulation of mucopolysaccharides in the skin.
To conclude that the dog had hypothyroidism, several tests were performed. After the atrial fibrillation was confirmed, a complete blood cell count revealed moderate leukocytes. A serum biochemical profile showed increased alanine aminotransferase and creatine kinase. The researchers also measured the levels of T3, T4, free thyroxine and free triiodothyronine. The results indicated that there was a decrease in the total thyroxine, triiodothyronine and free thyroxine and an increase in TSH. Upon analysis of the results, it was confirmed that they were consistent with severe primary hypothyroidism. It is evident from this study that many other problems accompany hypothyroidism which in turn affects several other body organs.
Thyroid dysfunction can result in numerous problems in other body organs due to little or lack of thyroid hormone which is responsible for metabolism (Flood & Hoover, 2009). The destruction of the thyroid gland causes hypothyroidism in most animal species especially canines. Hypothyroid dogs have been found to manifest numerous signs and symptoms that are as a result of failure of not only the thyroid gland but also other organs like the Central Nervous System (CNS), Reproductive organs, and the Skeletal system (The Merk Veterinary Manual, 2011). Thyroid hormones play a crucial role in ensuring the growth and development of the skeletal system as well as the CNS. This is particularly so during the fetal stage through the first few months of postnatal stage of life. Congenital as well as juvenile-onset hypothyroidism is largely responsible for the impairment of the CNS and the skeletal system (Bonham, 2005). They are characterized by disproportionate dwarfism, impaired cognitive development, and general loss of coordination in movement. Goiter is associated with hereditary hypothyroidism which is characterized by the swelling of the thyroid gland. The disorder causes a generally poor development of epiphyses in the entire length of the long bones, very short vertebral bodies, as well as delay in the closure of epiphyseal. In female dogs, hypothyroidism causes failure to cycle, infertility, premature births, and high litter mortality rate (Kustritz, 2009). Male dogs, on the other hand, lack libido, develop testicular atrophy/degeneration, infertility, and hypospermia. However, research has shown that hypothyroid male dogs can return to normal fertility after being put on proper thyroid hormone supplements (Kustritz, 2009).
Diagnosis
Hypothyroidism has been identified as one of the most common endocrinopathy in small animal practice. The diagnosis of this disorder can be quite challenging because most clinical signs of other diseases as well as conditions may be confused with those of hypothyroidism (Gross, 2005; Flood & Hoover, 2009). The administration of exogenous thyroid hormone can help improve these signs in dogs. There is no single biochemical test that may be used to diagnose hypothyroidism. Hormonal tests that may be performed must be interpreted with both historical and physical findings in mind. A veterinarian would first review the dog’s history and /or carry out physical examinations to determine the possibility of hypothyroidism. As a preliminary diagnosis, blood tests are usually performed (Coile, 2009). This initial tests help determine the levels lipids and cholesterol in blood. High levels of both are strong indicators of hypothyroid disorder. According to research findings, about 50% of dogs suffering from hypothyroidism also suffer from anemia (Case, 2005). Therefore, this condition is also usually checked during the diagnosis process.
To establish a definitive judgment, according to Gross, specific testing of thyroid function must be performed especially on the various signs linked to hypothyroidism (2005). Diagnosis of hypothyroidism in dogs is more accurate when thyroid tests are carried out. If the level of TSH is high while those of T4 and fT4 are low, then there is high likelihood that the dog could be suffering from the disorder (The Merk Veterinary Manual, 2011). The same article points out that a special TSH stimulation test may be conducted to confirm a diagnosis of thyroid disorder. When this TSH stimulation is done, the levels of T4 will not increase if the dog is suffering from this condition.
Despite the numerous tests available for the diagnosis of the endocrine disorder in canines, the process has never been easy and relying on a single test has resulted in over-diagnosis/misdiagnosis of hypothyroid disease (Mecklenburg et al., 2009). Research findings have demonstrated that the accuracy of a given diagnostic test is influenced by drugs, obesity, as well as other medical conditions. Any veterinarian must know the effects of drugs since it will facilitate correct interpretation of results and reduce incidences of erroneous diagnosis of hypothyroid disorder. It has been recommended that a combination of multiple tests usually increases the accuracy of diagnosing hypothyroidism (Daminet and Ferguson, 2003). In the veterinary field, there are a number of tests used in the diagnosis of thyroid disease. They include total thyroxine (TT4), free T4 (fT4),total 3,5,3’-tri-iodothyronine (TT3), endogenous canine thyroid stimulating hormone (cTSH), TSH response test, TRH response test, T4 and T3 autoantibodies, antithyroglobulin antibodies, nuclear scintigraphy, and thyroid gland biopsy (Coile, 2009). The most commonly preferred tests are TT4, fT4 by dialysis, and cTSH.
According to an article by Foster and Smith Educational Staff (2010), a number of diagnostic tests may be used to diagnosis hypothyroidism. The first is the Baseline T4 test which is one of the most common. A blood sample is obtained from the dog by a veterinarian and the levels of T4 thyroid hormone in the blood are determined by a process known as radioimmunoassay. Thyroid gland is the only organ that produces T4 hormone. Dogs with malfunctioned thyroid gland will have very low levels of T4 hormone which is a strong indicator of hypothyroidism. To determine the decreased level of T4, the veterinarian must have prior knowledge of the normal T4 level of the suspected hypothyroid canine. However, the weakness of this particular test is that low levels of T4 may be due to other causes other than hypothyroidism. This implies that once the test is positive for the endocrine disorder, another more discriminating test should be done for confirmation purposes.
The second test identified is Baseline T3 which can be run on a suspected hypothyroid dog. As mentioned earlier, T3 is a type of thyroid hormone circulating in the bloodstream. This test may be used alternatively with that of T4 when performing a screening test. The major difference in these two tests, however, is that T3 is not reliable when it comes to detecting early cases of the disorder. Occasionally, the levels of T3 will be found to be normal while a T4 test records reduced levels. The only appropriate time when this test can be used is together with TSH level or TSH stimulation test.
Another diagnostic test discussed in the article is free T4 (fT4) by equilibrium dialysis. The T4 hormone exists in the body in two forms. The first is commonly referred to as the “bound” form. It is attached to proteins found in the blood and is therefore not able to penetrate cells. The “free” T4 hormone is the other form and, unlike the T4, it is not connected to proteins in blood. This makes it easy to infiltrate into cells and perform its functions. The amount of fT4 in the bloodstream is usually quite small. However, advances in veterinary medicine have seen the development of equilibrium dialysis which is a special laboratory test that can accurately measure the fT4 in the blood.
The fourth test employed in the diagnosis of hypothyroidism is the TSH level. It is a blood test that determines the amount of TSH circulating in the bloodstream. In a dog suffering from hypothyroid disorder, the TSH level will be high due to the reaction of the body is trying to stimulate the thyroid gland to release more thyroid hormone needed for the proper functioning of other organs in the body, especially for metabolism. Once it is found that the levels of the Baseline T4 as well as Baseline T3 are much lower than normal and the TSH level is significantly high, then a veterinarian can conclude that the dog is hypothyroid.
The fifth and the last diagnostic test discussed in the article is the TSH Stimulation which is a reliable hypothyroidism confirmation test when it has already been established that the levels of T4 and T3 are low. It is performed by injecting a small amount of TSH into the vein. The level of T4 is in the blood is then determined after six hours. A dog with no thyroid disorder but with other diseases that may cause a lot of T4 will register high levels of T4 after the procedure. When the level of T4 thyroid hormones remains relatively constant after the TSH is injected, the hypothyroidism test is positive. The results showing changes in the concentration of thyroid hormone(T4) should be interpreted while taking into consideration the history as well as the clinical signs exhibited by the dog especially the one that is under medication (Daminet & Ferguson, 2003). Some of the most important drugs that influence diagnosis include glucocorticoids, Phenobarbital, sulfonamides, propranolol, potassium bromide and carprofen. Another important factor to consider during diagnosis is the duration of treatment and the amount of drug dosage. Daminet and Ferguson (2003) note that the mechanisms by which the drugs interfere with the functioning of the thyroid are yet to be understood fully. Generally, there are other diagnostic tests for hypothyroidism but they are usually effective when used in humans but not in canines.
Treatment of Hypothyroidism
This is one of the most critical stages when it comes to caring for animals. It is strictly required that only licensed veterinarians perform the treatment of animals (Coile, 2009). They should also update their knowledge by consulting the latest literature and newly developed pharmacological formularies before any treatment protocol is initiated (Bell et al., 2005). Although hypothyroidism disorder may be difficult to diagnose, it is one of the easiest treatable diseases. According to the article by Foster and Smith Educational Staff (2010), a hypothyroid canine (dog) is placed on a dose of a synthetic thyroid hormone known as thyroxine (levothyroxine) which is to be administered daily for the rest of its life. This helps in the replacement of the reduced or missing thyroid hormone. The drug comes under a number of brand names. Normally, the severity of the hypothyroid disorder determines the dosage frequency. The other determinant of drug dosage is the response to the drug by individual dog. The prescription of the drug takes into account the animal’s weight, usually 0.02 mg/kg orally at intervals of 12 hours (Bell et al., 2005; Day, 2008). Once the dog has been put on medication, the veterinarian takes blood samples periodically to monitor the response and adjustment to the dose made accordingly.
Once the treatment has been made for sometime, most of the symptoms of hypothyroidism disappear but this can only be sustained as long as the dog is under medication. A good example is the 8-year-old, male, Alaskan malamute whose myocardial function and improvement in facial myxedema as well as effusions resolved with the administration of levothyroxine (Flood & Hoover, 2009). According to Bell and others, clinical improvement is the most reliable indicator of the effectiveness of the therapy (2005). These include improvement of metabolic signs like mental dullness and lethargy within a fortnight since the commencement of the therapy. Some abnormalities especially dermatologic signs, however, may take relatively longer to resolve. The overall effect of thyroid hormone replacement treatment is notable decrease in the concentration of cholesterol and triglyceride in the bloodstream as well as an increase in the red blood cell (RBC) count (Bell et al., 2005). However, this indicator is not reliable for confirming the efficacy of the therapy. The best way to treat hypothyroidism in canines is by regularly doing blood tests. This will ensure that early detection is made and medication initiated before it reaches the chronic stages where all manifestations explode (Volhard & Volhard, 2011). Other preventive measures include feeding the dogs and cats on proper diet, avoiding excessive spraying using potentially harmful chemicals, and running blood tests on a puppy before acquisition to eliminate chances of congenital hypothyroidism.
Conclusion
The research paper has elaborately discussed the causes, signs and symptoms of hypothyroidism disorder, and the organ systems that this disease could harm. The various diagnostic procedures for hypothyroid dogs have also been discussed. The functioning of the organ systems including the nervous system, cardiovascular, and skeletal is severely influenced by hypothyroidism, the most common hormone deficiency disease in dogs. Thyroid hormone plays a significant role in the body especially metabolic processes which are required by the entire body. It has also emerged from the above discussion that hypothyroidism is one of the most difficult disease to diagnose and easier to treat. The T4, T3, free T4 (fT4), TSH level, and TSH Stimulation tests are some of the diagnostic methods that can be used to determine the presence of hypothyroidism. The paper has pointed out that there is no single test for hypothyroidism but a combination of two or more can lead to accurate diagnosis. However, research on advanced diagnostic alternatives for hypothyroidism is ongoing to develop a single-test procedure for diagnosing hypothyroidism. Levothyroxine is a very common treatment option for hypothyroid dogs. The dosage of the treatment depends on the age, weight, and severity of the condition of the hypothyroid dog. Among the canines, hypothyroidism disorder is more prevalent among dogs. The main causes of hypothyroidism are genetically induced or due to an autoimmune system attack on the thyroid gland. The paper has also discussed the two most known causes of hypothyroidism disorder. This disorder must be given the attention it deserves due to the vast impact it has on other body organs. The good news with this disease is that once it is properly diagnosed and therapy started, the dog is not at risk of dying.
References
Bell, E., Latimer, K. S., LeRoy, B. E., & Moore, H. (2005). Canine hypothyroidism: an overview. Veterinary Clinical Pathology Clerkship Program
Bonham, M. H. (2005). Bring me home! Dogs make great pets. John Wiley and Sons
Case, L. P. (2005). The dog: its behavior, nutrition, and health (2nd ed.). Wiley-Blackwell
Coile, C. (2009). The golden retriever handbook (2nd ed.). Barron’s Educational Series
Daminet, S. & Ferguson, D. C. (2003). Influence of drugs on thyroid function in dogs. Journal of Veterinary Medicine, 17: 463-472. Web.
Day, M. J. (2008). Clinical immunology of the dog and cat (2nd ed.). Manson Publishing.
Flood, J. A. & Hoover, J. P. (2009). Improvement in myocardial dysfunction in a hypothyroid dog. Canadian Veterinary Journal, 50: 828-834. Web.
Foster & Smith Educational Staff (2010). “Thyroid Diseases: Hypothyroidism in Dogs.” Web.
Gross, T. L. (2005). Skin diseases of the dog and cat: clinical and histopathologic diagnosis (2nd ed.). Wiley-Blackwell
Kustritz, M. V. R. (2009). Clinical canine and feminine reproduction: evidence-based answers. John Willey and Sons
Mecklenburg, L., Linek, M. & Tobin, D. J. (2009). Hair loss disorders in domestic animals. John Wiley and Sons
PetWellbeing (2011). “Dog Hypothyroidism (Canine Hypothyroidism.” Web.
The Merk Veterinary Manual (2011). Hypothyroidism. Web.
Volhard, J. & Volhard, W. (2011). Dog training for dummies (2nd ed.). For Dummies
Ward, C. R. (ed) (2007). The thyroid. Veterinary Clinics of North America Small Animal Practice. WB Saunders Co., Philadelphia PA