Thyroid Function

Other common endocrine conditions include diseases of the thyroid gland. The ones which are most common/heard about are hypothyroidism usually affecting dogs and hyperthyroidism usually affecting cats. 

Hypothyroidism

• Especially seen in old dogs. 

Primary hypothyroidism

Causes include:

• Loss of functional parenchyma (loss of functioning thyroid gland) due to aplasia (organ doesn't develop), hypoplasia (incomplete development), atrophy (decrease in organ size after full growth), inflammation and neoplasia. All of these factors destroy the tissue which results in a decreased capacity for the thyroid to produce the sufficient amounts of the hormones it needs.
• Deficiency of hormone components especially iodine. For hormones to be released into the blood, they are bound in a substance known as colloid in the gland by thyroglobulin. Iodine is needed to bind the tyrosine residues (thyroid hormone) in place for endocytosis (excretion). Therefore a lack of iodine will disrupt the release of thyroid hormones.
• Chemical blockage of production e.g. thioureate.
• Gland resection. 

Canine hypothyroidism is normally due to idiopathic follicular collapse or lymphocytic thyroiditis (inflammation). 

The thyroid gland hormones are responsible for the basal metabolic rate Therefore a reduction in the hormones results in a decreased basal metabolism which results in the following consequences:

• Increased body weight- due to decreased metabolism.
• Dermatological- thin coat, alopecia with hyperkeratosis and hyperpigmentation.
• Reproduction- abnormal oestrus cycles and a reduced sperm count.
• Goitre- enlarged thyroid seen with iodine deficiency. 
• Myxoedema- mucoid oedema in the subcutis/mucosa seen in fetal hypothyroidism.
• Cretinism- dwarfism seen with endemic iodine deficiency or endogenous cretinism (sporadic in the goat). 

Secondary hypothyroidism

• Pituitary gland TSH deficiency. 

Hyperthyroidism

• Common in aged cats with thyroid multinodular hyperplasia and adenoma.

Consequences of increased basal metabolism:

• Weight loss.
• Hepatic lipidosis and necrosis due to glycogen deficiency. 
• Left ventricular hypertrophy. 

Other Thyroid Disorders

• Inflammation- chronic lymphocytic thyroiditis in the dog. An autoimmune disorder.

Hyperplasia

Known as goitre as a consequence of thyroid hormone deficiency. Can also happen after partial gland resection for compensatory mechanism. Occurs due to:

• Hyperthyroid leads to excessive levels of thyroid hormone in circulation which inhibits TRH and TSH secretion leading to atrophy of the normal parenchyma. 
• Hypothyroid leads to functional hyperplasia to try to maintain function. Can also occur in  due to extreme iodine deficiency or with herbivore congenital disease associated goitrogenic plants. 
• Euthyroid is the most common cause of functional hyperplasia. It is due to endemic goitre deficiency. 

Appears diffuse in young animals and nodular in old animals. 

Neoplasia

 

Epithelial Tumours 

• Frequent in areas of endemic goitre.
• Adenoma-> seen in the cat and produce excess thyroid hormone.
• Carcinoma-> early metastasis haematogenously to the lung. Follicular, papillary or solid growth. 

Mesenchymal Tumours

• Fibroma etc. 

C-Cell Tumours 

• Tumour of the cells that produce the hormone calcitonin. Rare and sometimes seen in old animals.