Thyroid Gland - Pathology

()Map ENDOCRINE SYSTEM (Map)

Anatomy and Physiology

Anatomy and Physiology of the thyroid gland can be found here.

Goitre

Refers to a benign enlargement of the throid gland. May be hypofunctional or hyperfunctional. (More often see increased function with neoplasia of the thyroid or the pituitary glands).

Diffuse parenchymatous goitre

Hypofunctional goitre, low levels of thyroglobulin is produced.

Parenchymatous goitre. Courtesy of A. Jefferies

Aetiology:

• Dietary iodine deficiency.
• Goitrogenic compounds E.g. Thiocyanates. Reduce uptake of iodine by the thyroid. Includes rape, kale and white clover.

Most often seen as a congenital disease in areas with iodine deficient soil.

Clinical signs:

• Lamb/calf/piglet born dead or moribund.
• Hairless.
• Oedema of head and neck.

Pathophysiology:

No thyroid hormones are produced and there is overstimulation of the thyroid gland by TSH as there is no negative feedback. Histopathologically seen as hyperplasia of the epithelial lining cells as they are stimulated by TSH. Very little colloid is formed as no iodine is available and the epithelial cells form papillary ingrowths into the follicle centre.

Diffuse colloid goitre

Also hypofunctional. Failure at the pituitary level to produce TSH. TSH is needed for the release of thyroglobulin so without it colloid will accumulate and the follicles will become inactive, seen as:

Colloid goitre. Courtesy of A. Jefferies

• Large amounts of densely staining colloid.
• Flattened, low cuboidal epithelium.
• Follicles may coalesce to form cystic spaces.

Usually due to damage to the TSH producing cells of the pituitary.

Nodular goitre

Results from a cycle of hyperplasia and hypoplasia.

Hyperthyroidism

Seen most commonly in the cat. Elderly cats are affected with the average age of onset being 12-13 years.

Clinical signs:

• Weight loss despite polyphagia
• Increased activity, nervousness (approximately 10% will show apathy)
• Polyuria and polydipsia
• Heat intolerance, panting
• Tachycardia >240 bpm
• Poor coat, matted and unkempt
• Palpable thyroid mass often present

Often see a hypertrophic cardiomyopathy due to chronically increased heart rate and activity.

Thyroid adenoma. Courtesy of T. Scase

Aetiopathogenesis:

>98% are a functional adenoma of the thyroid gland and many will be palpable.

Treatment:

Surgical removal of the affected thyroid gland(s). If bilateral it is important to preserve at least one parathyroid gland to maintain calcium homeostasis.

Medical treatment:

• Carbamizole: Interferes with thyroid hormone synthesis.
• Radioactive iodine therapy. Useful for intrathoracic thyroid nodules.