HR Lesson 3: Pathogenesis and evaluation of the thyroid gland

Neoplasia form glandular tissues

Adenoma (benign) Carcinoma (malignant)

Adenomas

Benign

slow growing and locally

do not metastasize

Carcinomas

Malignant

Grow rapidly

Often spread to other organs

Dogs get which type of thyroid tumors usually?

Carcinomas, usually unilateral and nonfunctional

Cats get which type of thyroid tumors usually?

Adenomas, usually bilateral and function

What does a functional thyroid gland tumor mean?

Has the ability to manipulate the thyroid gland and hormone production

What are the 3 possible differentials for thyroid pathology?

Hyperthyroidism, hypothyroidism, and euthyroid sick syndrome

In dogs, hypothyroidism is usually caused by what?

Lymphocytic thyroiditis

Lymphocytic thyroiditis

Lymphocytes invade thyroid gland and destroy it, autoimmune, lowers T4

In cats, hyperthyroidism is usually caused by what?

Adenoma

Euthyroid Sick syndrome

Non-thyroidal illness that depletes thyroid hormones and gland usually from a systemic illness or chronic medication

Mechanism of euthyroid sick syndrome

Protective response to reduce basal metabolic rate (BMR) during stress/illness

Suppresses thyroid hormone secretion to conserve energy

Causes of euthyroid sick syndrome

Systemic illness: trauma, stress, etc

Medications

Physiological factors (age, hormonal cycles, etc)

How to test for euthyroid sick syndrome

Drug withdrawal (6 weeks) needed before testing to avoid misdiagnosis

Primary hypothyroidism (95% cases)

Thyroid gland disfunction that decreases T4

Lymphocytic thyroiditis: immune mediated destruction by lymphocytes, plasma cells, and macrophages

>60% gland destroyed before signs, progressing over 1-3 years with fibrosis

Other causes of hypothyroidism

Secondary hypothyroidism (<5% cases): pituitary dysfunction

Neoplasia —> often non-functional (carcinoma)

Hyperthyroidism

Common in cats —> primarily > 9 yrs old

Cause of hyperthyroidism

Thyroid adenomas (75% bilateral)

Rare in dogs

What do thyroid adenomas cause?

excess T4 production leading to hyperthyroidism

Genetic predispositions of lymphocytic thyroiditis

Beagles and Doberman Pinschers