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.
Classification 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.
Causes The causes of primary hypothyroidism include: 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.
The causes of secondary hypothyroidism include: 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: Congenital hypothalamic malformation Acquired hypothalamic destruction Defective thyrotropin-releasing hormone molecules Defective thyrotropin-releasing hormone-thyrotropin receptors
Primary hypothyroidism Lymphocytic thyroiditis 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 activate the process that causes follicular destruction. Breed is an important factor given the increased incidence in certain breeds, such as the Beagle.
Neoplastic destruction 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, in part because most thyroid tumours do not affect the whole thyroid gland.
Idiopathic atrophy Idiopathic atrophy is 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.
Iodine deficiency Iodine deficiency is a rare cause of hypothyroidism in dogs: commercial dog foods typically contain sufficient levels of iodine for dogs. In recent years, there has been increased interest in homemade diets for pets; however, these diets do not always provide sufficient iodine levels. 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.
Other causes Goitrogens and anti-thyroid medication are potential causes of primary hypothyroidism, but are rare in practice. Sulfonamides inhibit thyroid peroxidase and can cause hypothyroidism via that mechanism.
Thyroidectomy may cause hypothyroidism: in one study, more than 50% of dogs that underwent a bilateral thyroidectomy required long-term thyroid hormone treatment.
Radioactive iodine used to treat thyroid neoplasia can also result in hypothyroidism.
Secondary hypothyroidism Secondary hypothyroidism occurs when pituitary thyrotrophs fail to develop, resulting in impaired thyroid-stimulating hormone secretion. Inadequate levels of thyroid-stimulating hormone decrease thyroid hormone synthesis and secretion, leading to hypoplasia of the thyroid gland.
Secondary hypothyroidism caused by naturally acquired defects of thyrotrophs, like those resulting from a neoplasia, is uncommon. Secondary hypothyroidism caused by thyrotropin suppression from a comorbidity, malnutrition, hormones, or drugs is a prevalent cause of secondary hypothyroidism.
Pituitary destruction The destruction of thyrotrophs by a pituitary neoplasia may cause secondary hypothyroidism, although pituitary neoplasias do not commonly cause secondary hypothyroidism.
Tertiary hypothyroidism Tertiary hypothyroidism is caused by an inadequate amount of thyrotropin-releasing hormone being secreted by the hypothalamus. Thyrotropin-releasing hormone stimulates the release of thyroid-stimulating hormone; a lack of thyrotropin-releasing hormone leads to decreased thyroid-stimulating hormone, which causes follicular atrophy.
Congenital hypothyroidism Congenital hypothyroidism is a rare condition that is often underdiagnosed because dogs with the condition often die 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 are typically shown within the first 3 months.
Signs and symptoms Signs of hypothyroidism vary based on factors such as the type and breed. Due to the slow onset and progression of the disease, clinical signs do not initially appear; for example, they do not appear until the 3rd stage of lymphocytic thyroiditis. It is not uncommon for owners to ignore certain signs of hypothyroidism due to the slow progression of metabolic symptoms. Often, metabolic symptoms will go unnoticed until after a dog has received treatment for hypothyroidism.
The most common signs of hypothyroidism are dermatological and changes related to decreased metabolism. In one study, the most common symptoms were dermatological conditions, obesity, and lethargy. Nearly 9/10 dogs had dermatological symptoms, and lethargy and obesity were observed in almost half of all dogs.
A lowered concentration of thyroid hormone affects organs throughout the body.
Dermatological 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.
Myxoedema, also known as cutaneous mucinosis, occurs when hyaluronic acid builds up in the dermis. This buildup 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.
Neurological Hypothyroidism can affect the peripheral and central nervous systems. Neurological symptoms vary widely, and it is not currently known what causes them, 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.
Reproductive 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 to rule it out. It is recommended that when common causes of poor reproductive ability have been ruled out, hypothyroidism be considered as a possible cause. Hypothyroidism can result in excessive prolactin secretion due to increased thyrotropin-releasing hormone levels. Excessive prolactin secretion causes hyperprolactinaemia and may cause improper lactation in unspayed bitches.
Cardiovascular 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 are not a cause for concern 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 thyroid hormone deficiency. Thyroid hormone treatment usually results in resolution of cardiac abnormalities, although this may take several months.
Myxoedema coma Myxoedema coma is a very rare but deadly form of severe hypothyroidism characterised by myxoedema, asthaenia, hypothermia, bradycardia, 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 the 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.
Endocrine Due to the effect thyroid hormone has on the pituitary gland, hypothyroidism can cause hypersecretion of growth hormone.
Risk factors Hypothyroidism symptoms are usually diagnosed in older dogs; this may be in part due to the slow onset of symptoms.
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.
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%).
No evidence exists of a predilection for any sex, nor whether the animal is neutered or not.
Congenital hypothyroidism 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 symptoms are due to thyroid hormones' role in bone and mental growth.
Thyrotoxicosis Thyrotoxicosis is the presence of an abnormally high circulating thyroid hormone level; in this context, it does not refer to hyperthyroidism but instead to excessive levels due to thyroid hormone treatment. Thyrotoxicosis is a rare side effect in dogs receiving levothyroxine treatment due to the half-life and poor absorption. Thyrotoxicosis occurs when a dog is given too much levothyroxine or has difficulty metabolising it. Symptoms include: polypynoea, anxiety, tachycardia, aggression, polyuria, polydipsia, polyphagia, and cachexia.
Comorbidities