Draft:Hypothyroidism in dogs
| Hypothyroidism | |
|---|---|
 | |
| Labrador Retriever with hypothyroidism | |
| Specialty | Endocrinology |
| Differential diagnosis | Hypersomatotropism |
Hypothyroidism is an endocrine disorder in which the thyroid gland fails to produce sufficient thyroid hormones. Hypothyroidism is one of the most common endocrinopathies in dogs. It is either acquired or congenital.[1]
Classification[edit]
Hypothyroidism is classified as either primary, secondary, or tertiary. Primary hypothyroidism is for when the cause is due to an abnormality of the thyroid gland, secondary hypothyroidism is when the cause is decreased thyroid-stimulating hormone levels, and tertiary hypothyroidism is when the cause is an inadequate amount of thyrotropin-releasing hormone being released.[1]
Causes[edit]
The causes of primary hypothyroidism include:[1]
- Lymphocytic thyroiditis
- Idiopathic atrophy
- Neoplasia destroying the thyroid gland
- Iodine deficiency
- Ingestion of goitrogen
- Iatrogenic
- Leishmaniasis
- Congenital hypothyroidism
Lymphocytic thyroiditis and idiopathic atrophy are the most common causes of primary hypothyroidism.[1]
The causes of secondary hypothyroidism include:[1]
- Pituitary malformation
- Pituitary cyst
- Neoplasia destroying the pituitary gland
- Defective thyroid-stimulating hormone molecules
- Defective thyroid-stimulating hormone-follicular cell receptors
- Iatrogenic
The causes of tertiary hypothyroidism include:[1]
- Congenital hypothalamic malformation
- Acquired hypothalamic destruction
- Defective thyrotropin-releasing hormone molecules
- Defective thyrotropin-releasing hormone-thyrotroph receptors
Primary hypothyroidism[edit]
Lymphocytic thyroiditis[edit]
Lymphocytic thyroiditis is caused by an immune-mediated response to the thyroid gland. The exact mechanism is not known and the understanding of it is poor; however, it is believed that antibodies binding to the follicular cells, colloid, or thyroglobulin antigens activates the process that causes follicular destruction. Breed is an important factor given the increased incidence in certain breeds, such as the Beagle.[1]
Neoplastic destruction[edit]
After a tumour—either one originating in the thyroid gland or invading it—has destroyed 80% or more of the thyroid gland, symptoms of hypothyroidism may occur. Only approximately 10% of thyroid tumours cause hypothyroidism, this in part due to most thyroid tumours not affecting the whole thyroid gland.[1]
Idiopathic atrophy[edit]
Idiopathic atrophy is the the gradual reduction in the size of thyroid follicles and replacement of these follicles with adipose tissue. It may be the end stage of lymphocytic thyroiditis or a primary degenerative disorder.[1]
Iodine deficiency[edit]
Iodine deficiency is a rare cause of hypothyroidism for dogs: commercial dog foods typically contain sufficient levels of iodine for dogs. In recent years there has been an interest in homemade diets for pets, these diets are not always providing sufficient levels of iodine. Working dogs fed all meat diets have been reported to suffer from hypothyroidism due to iodine deficiency. In addition to iodine deficiency, excessive amounts of iodine can cause thyroid impairment and lead to hypothyroidism.[1]
Other causes[edit]
Goitrogens and anti-thyroid medication is a potential cause of primary hypothyroidism, but is rare in practice.[1]
Thyroidectomy may cause hypothyroidism: in one study more than 50% of dogs that underwent a bilateral thyroidectomy required long term thyroid hormone treatment.[2][1]
Radioactive iodine used to treat thyroid neoplasia can also result in hypothyroidism.[1]
Secondary hypothyroidism[edit]
Secondary hypothyroidism occurs when pituitary thyrotrophs fail to develop, resulting in impaired thyroid-stimulating hormone secretion. Inadequate levels of thyroid-stimulating hormone causes a decrease in thyroid hormone synthesis and secretion and cause hypoplasia of the thyroid gland.[1]
Secondary hypothyroidism caused by naturally acquired defects of thyrotrophs, like those resulting from a neoplasia, are uncommon. Secondary hypothyroidism caused by thyrotroph suppression from a comorbidity, malnutrition, hormones or drugs is a very common cause of secondary hypothyroidism.[1]
Pituitary destruction[edit]
The destruction of thyrotrophs by a pituitary neoplasia may cause secondary hypothyroidism, although pituitary neoplasias do not commonly cause secondary hypothyroidism.[1]
Tertiary hypothyroidism[edit]
Tertiary hypothyroidism is caused by an inadequate amount of thyrotropin-releasing hormone being secreted by the hypothalamus. Thyrotropin-releasing hormone stimulates release of thyroid-stimulating hormones, thus a lack of thyrotropin-releasing hormone causes a decrease in thyroid-stimulating hormone which causes follicular atrophy.[1]
Congenital hypothyroidism[edit]
Congenital hypothyroidism is a rare and is often underdiagnosed due to dogs with the condition often dying as young puppies. Congenital hypothyroidism is caused by a defect in the hypothalamic–pituitary–thyroid axis or the thyroid hormone receptor. It is not uncommon for puppies with the condition to die early due to retarded growth and dwarfism. Signs of congenital hypothyroidism typically shown within the first 3 months.[1]
Signs and symptoms[edit]
Signs of hypothyroidism vary based on factors such as the type and breed. Due to the slow onset and progress of the disease clinical signs do not initially appear for example, clinical signs do not show until the 3rd stage of lymphocytic thyroiditis. It isn't uncommon for owners to ignore certain signs of hypothyroidism due to the slow progression of metabolic symptoms. It's not uncommon for metabolic symptoms to go unnoticed until after a dog has received treatment for hypothyroidism.[1]
The most common signs of hypothyroidism are dermatological and changes relating to the decreased metabolism.[1] In one study the most common symptoms were dermatological conditions, obesity, and lethargy. Nearly 9/10 dogs had dermatological symptoms, lethargy and obesity were observed in almost half of all dogs.[3]
The lowered concentration of thyroid hormone has an affect on organs throughout the body.[4]
Dermatological[edit]
The decrease in thyroid hormone levels causes dermatological conditions, specifically alopecia, which is the most common dermatological symptom, as the hair sheds but cannot regrow fast enough due to the anagen stage of hair growth being stunted as thyroid hormone is important in that stage of hair growth. Other dermatological symptoms include: hyperkeratosis, hyperpigmentation, seborrhoea, pyoderma, otitis externa, and myxoedema. Breed variation in coats can impact the dermatological effect of hypothyroidism. Some dogs develop hypertrichosis and some develop either a wool-like or a coarse appearance. Immune dysregulation, caused by hypothyroidism, can lead to superficial bacterial infection.[1]
Myxoedema, also known as cutaneous mucinosis, occurs when hyaluronic acid builds up in the dermis, this build up may cause swelling and thickening of the skin. Myxoedema is a feature of severe cases of hypothyroidism and is sometimes used synonymously. The characteristic facial expression of hypothyroidism is in part caused by myxoedema.[1]
Neurological[edit]
Hypothyroidism can affect the peripheral and central nervous system. Neurological symptoms vary widely and it is not currently known what is responsible for causing neurological symptoms, although multiple theories have been put forward. Some of the other proposed causes of neurological symptoms include: nerve entrapment, due to mucinous deposits; demyelination, due to disruptions to the metabolism of Schwann cells; vascular nerve damage, due to changes to the blood-nerve barrier; and disruption of the axonal cell transport. An immune-related cause has been put forward as well, due to the presence of immune-mediated thyroiditis in some dogs with hypothyroidism.[1]
Reproductive[edit]
There is a belief of an association between hypothyroidism and reproductive symptoms causing poor reproductive ability. Studies have not identified this association; however, there is no definitive evidence from studies that can rule out an association. It is recommended that when common causes of poor reproductive ability have been ruled out that hypothyroidism be considered as a possible cause. Hypothyroidism can result in excessive prolactin secretion due to increased thyrotropin-releasing hormone levels. Excessive prolaction secretion causes hyperprolactinaemia and may cause improper lactation in unspayed bitches.[1]
Cardiovascular[edit]
It is uncommon for cardiovascular symptoms to be diagnosed during a clinical exam. The cardiovascular symptoms that are most often diagnosed during examination are bradycardia and a deficit of the apical impulse. The exact mechanism behind the cause of cardiovascular and haemodynamic symptoms is unknown but may be due to the effects of hypothyroidism on the cardiac muscle. Most cardiac abnormalities in dogs with hypothyroidism are typically mild and thus is not a cause for concern aside outside of long surgical procedures. Heart failure may occur in cases of primary hypothyroidism; however, heart failure caused by hypothyroidism is rare, most cases of heart failure are a preexisting condition being exacerbated by the thyroid hormone deficiency. Thyroid hormone treatment usually results in an a cessation of cardiac abnormalities, although it may take several months for this to occur.[1]
Myxoedema coma[edit]
Myxoedema coma is a very rare but deadly form of severe hypothyroidism characterised by myxoedema, asthaenia, hypothermia, bradycardia and organ dysfunction, hypotension, hypoventilation and unresponsiveness. It may swiftly progress to stupor and coma. Dogs with the condition have similar test results as those with normal hypothyroidism but prognosis is very poor and most dogs will die from organ failure or an infection occurring alongside the condition. The condition often occurs after a bacterial infection or a bout of hypothermia.[1]
Endocrine[edit]
Due to the effect thyroid hormone has on the pituitary gland hypothyroidism can cause hypersecretion of growth hormone.[5]
Risk factors[edit]
Hypothyroidism symptoms are usually diagnosed in older dogs, this may be in part due to the slow onset of symptoms.[1]
Breed is a factor in the progression and likelihood of thyroiditis. Golden Retrievers and Doberman Pinschers are often reported as having a predilection for the condition. Breeds with a described mutation in the TPO gene include: Toy Fox Terriers, Rat Terriers, Tenterfield Terriers, and Spanish Water Dogs. Environmental factors have not been well described in the literature.[1]
In a study of more than 140,000 serum samples of suspected hypothyroidism, the five breeds with the highest prevalence of thyroglobulin autoantibodies were: English Setter (31%), Old English Sheepdog (23%), Boxer (19%), Giant Schnauzer (19%), and American Pit Bull Terrier (19%).[6]
No evidence exists of a predilection for any sex nor whether the animal is neutered or not.[1]
Congenital hypothyroidism[edit]
Congenital hypothyroidism has unique symptoms that distinguish it from other forms of hypothyroidism such as: dwarfism, growth retardation, kyphosis, retarded tooth eruption, shortened lower jaw, and an enlarged skull. The cause of the symptoms are due to the rule of thyroid hormone in bone and mental growth.[1]
Comorbidities[edit]
Immune-mediated endocrine disorders such as diabetes mellitus and hypoadrenocoriticism often occur alongside lymphocytic thyroiditis. One study found 4% of dogs with hypoadrenocorticism also had hypothyroidism and one dog had hypoadrenocorticism, hypothyroidism, diabetes mellitus, and hyperparathyroidism. Orchitis occurring alongside thyroiditis has been documented in Beagles. The average time for a diagnosis of a second endocrinopathy is 4 months, with one dog having a period of 53 months between diagnosis.[1]
A pituitary tumour that causes pituitary destruction may cause hypothyroidism, alongside: hypocortisolism, diabetes insipidus, and sexual dysfunction.[1]
Pathophysiology[edit]
Lymphocytic thyroiditis[edit]
Lymphocytic thyroiditis is the infiltration of the thyroid gland by lymphocytes, plasma cells, and macrophages. The infiltration progressively causes destruction of follicles and secondary fibrosis. Clinical signs typically do not show before 80% of the thyroid gland has been destroyed. This destruction occurs over a long period of time, between 1-3 years. This destruction is classified into four stages: subclinical thyroiditis, when the infiltration of thyroid follicles occurs, test results will be positive for thyroglobulin and thyroid hormone autoantibodies; antibody positive subclinical hypothyroidism, when the loss of more than 60% of thyroid gland mass has occurred and in response there is an increase of thyroid-stimulating hormone, this stimulates the thyroid gland to maintain thyroxine levels; antibody positive overt hypothyroidism, when a majority of the functional thyroid tissue has been destroyed, test results will show a decrease in serum thyroid hormone concentration and an increase in thyroid-stimulating hormone concentration; and noninflammatory atrophic hypothyroidism, when the thyroid tissue has been replaced by fibrous and adipose tissue and a disappearance of inflammatory cells and circulating antibodies has occurred.[1]
Diagnosis[edit]
Lymphocytic thyroiditis[edit]
Lymphocytic thyroiditis can be diagnosed as the cause of hypothyroidism via histological examination. Histological examination will reveal mononuclear cell infiltration, the absence of follicles containing colloid, and a change in the normal appearance and layout of the cells. Proteinuria may be identified in dogs with lymphocytic thyroiditis but otherwise urinalysis results should be normal.[1]
Idiopathic atrophy[edit]
There are no tests for idiopathic atrophy and due to that it is made via diagnosis of exclusion. Negative results on tests for other forms and symptoms aligning with the condition are used to diagnose it.[1]
Secondary hypothyroidism[edit]
Secondary hypothyroidism can be histologically diagnosed by looking at the follicles of the thyroid gland, these follicles will by hypoplastic and lack in colloids in cases of secondary hypothyroidism. Testing will typically reveal decreased or absent levels of thyroid-stimulating hormone; however, this result does not confirm secondary hypothyroidism.[1]
Tertiary hypothyroidism[edit]
When tertiary hypothyroidism is suspected it can be confirmed via testing, if thyroid-stimulating hormone levels increase after thytropin-releasing hormones have been administered then a diagnosis of tertiary hypothyroidism has been confirmed. The sensitivity of the thyroid-stimulating hormone assays for dogs makes this is a difficult diagnosis.[1]
Differential diagnosis[edit]
Dogs being treated for epilepsy may be incorrectly diagnosed as having hypothyroidism as anticonvulsant medication used to treat epilepsy can alter the results of tests that measure thyroid hormone levels.[1]
Electrocardiography[edit]
Common electrocardiographic findings in dogs showing symptoms of hypothyroidism include sinus bradycardia and atrioventricular blocks.[1]
Echocardiography[edit]
Common echocardiographic findings in dogs showing symptoms of hypothyroidism include an increased systolic diameter of the left ventricle, a decreased thickness of the left posterior ventricular wall, and prolongation of the pre-ejection period when the heart muscle contracts; and changes to the left ventricular posterior wall, interventricular wall thickness, and diameter of the aorta.[1]
Complete blood count[edit]
A form of anaemia that is normocytic, normochromic, and non-regenerative is diagnosed in approximately 30% of dogs with hypothyroidism.[1]
Serum biochemistry[edit]
The most common finding in with a serum biochemistry panel is fasting hypercholesterolaemia, this finding appears in approximately 75% of dogs with hypothyroidism. Hypertriglyceridaemia is also a common finding. Neither of these are pathognomonic; however, they are supportive evidence of hypothyroidism. Increases in serum lactate dehydrogenase, aspartate aminotransferase, alanine aminotransferase, and alkaline phosphatase activity.[1]
Radiography[edit]
Radiography is not standard procedure for identifying acquired hypothyroidism. Cervical spine radiographs are only useful when neoplasia is suspected as the cause.[1]
Abnormalities can be detected via radiography in cases of congenital hypothyroidism and these include: retarded epiphyseal ossification; epiphyseal dysgenesis; brachycephalic skull, when untypical for the breed or shorter than expected; shortened vertebral body; reduced length of diaphyses of long bones; retarded ossification; and retarded maturation.[1]
Epidemiology[edit]
The most common type is acquired primary hypothyroidism which makes up approximately 95% of hypothyroidism cases in dogs. Secondary hypothyroidism is rare and tertiary hypothyroidism is even rarer.[1]
References[edit]
- ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af ag ah ai aj ak al am an ao ap aq ar as Scott-Moncrieff, J. Catherine (2015). "Hypothyroidism". In Feldman, Edward C.; Nelson, Richard W.; Reusch, Claudia; Scott-Moncrieff, J. Catharine (eds.). Canine and feline endocrinology (Fourth ed.). St. Louis, Missouri: Elsevier Saunders. pp. 77–128. ISBN 978-1-4557-4456-5.
- ^ Tuohy, Joanne L.; Worley, Deanna R.; Withrow, Stephen J. (2012-07-01). "Outcome following simultaneous bilateral thyroid lobectomy for treatment of thyroid gland carcinoma in dogs: 15 cases (1994–2010)". Journal of the American Veterinary Medical Association. 241 (1): 95–103. doi:10.2460/javma.241.1.95. ISSN 0003-1488.
- ^ DL, Panciera (2001). "Conditions associated with canine hypothyroidism". The Veterinary Clinics of North America. Small Animal Practice. 31 (5). Vet Clin North Am Small Anim Pract: 935–950. doi:10.1016/S0195-5616(01)50006-6. ISSN 0195-5616. PMID 11570133. Retrieved 2024-05-02.
- ^ Diaz-Espiñeira, M.M.; Galac, S.; Mol, J.A.; Rijnberk, A.; Kooistra, H.S. (2008). "Thyrotropin-releasing hormone-induced growth hormone secretion in dogs with primary hypothyroidism". Domestic Animal Endocrinology. 34 (2): 176–181. doi:10.1016/j.domaniend.2007.02.001.
- ^ Diaz-Espiñeira, M.M.; Mol, J.A.; van den Ingh, T.S.G.A.M.; van der Vlugt-Meijer, R.H.; Rijnberk, A.; Kooistra, H.S. (2008). "Functional and morphological changes in the adenohypophysis of dogs with induced primary hypothyroidism: Loss of TSH hypersecretion, hypersomatotropism, hypoprolactinemia, and pituitary enlargement with transdifferentiation". Domestic Animal Endocrinology. 35 (1): 98–111. doi:10.1016/j.domaniend.2008.03.001.
- ^ Graham, Peter A.; Refsal, Kent R.; Nachreiner, Raymond F. (2007). "Etiopathologic Findings of Canine Hypothyroidism". Veterinary Clinics of North America: Small Animal Practice. 37 (4): 617–631. doi:10.1016/j.cvsm.2007.05.002. PMID 17619002.