Pathology of the thyroid gland

Question 1

Composition of the thyroid gland

Answer 1

Two distinct lines connected by a narrow isthmus

Question 2

Histology of thyroid gland

Answer 2

Follicles lined by epithelial cells, and containing colloid.
Amount of colloid and appearance of lining reflects level of gland activity.

Second population of parafollicular medullary, or C cells, that produce calcitonin.

Question 3

How is thyroid hormone production regulated?

Answer 3

The hypothalamic-pituitary-thyroid axis.

Iodide taken up by follicle cells to for thyroxine (T4) and triiodothyronine (T3).

T4 and T3 released into blood or stored in colloid.

Low levels of plasma T4 and T3 stimulate expression of thyroid releasing hormone (TRH) by the pituitary.

Question 4

Thyroid goitre

Answer 4

A non-neoplastic and non-inflammatory clinical enlargement of the thyroid gland

Question 5

Factors causing hyperplasia of thyroid gland (goiter)

Answer 5

• iodine deficient diets (TSH induced goiter)
• goitrogenic compounds (interfere with thyroid hormone synthesis)
• dietary iodine excess (compensatory increase in TSH secretion)
• genetic enzyme defects (synthesis of thyroid hormones)

Question 6

Causes of canine hypothyroidism

Answer 6

• Lymphocytic thyroiditis (e.g. Hashimotos)
• Follicular atrophy (idiopathic)
• goiter (iodine deficient)
• neoplasia (bilateral non functional thyroid tumours)
• enzymatic defects

Question 7

Clinical signs of canine hypothyroidism

Answer 7

• lethargy
• weight gain
• alopecia
• etc.

Question 8

Most common causes of canine hypothyroidism

Answer 8

Immune mediated (lymphocytic) thyroiditis

Idiopathic follicular atrophy

Question 9

Thyroiditis

Answer 9

May be an autoimmune disease

Results in infiltrates of lymphocytes within gland

Produces antibodies to thyroglobulin, thyroperoxidase, and TSH receptor

Thyroid may be enlarged or atrophic (depends on stage)

Question 10

Idiopathic follicular atrophy

Answer 10

Follicles cease making colloid

gland gradually replaced by fibrinous or fatty connective tissue

Original cause of lesion unknown

Question 11

Hypothyroidism and cholesterol

Answer 11

Long standing canine hypothyroidism can be associated with marked hypercholesterolaemia

Can lead to secondary extrathyroidal lesions like atherosclerosis, hepatomegaly, renal glomerular lipidosis, and corneal lipidosis

Question 12

Endocrine skin disease resulting from hypothyroidism

Answer 12

Specific to hypothyroidism:
Myxoedema- increased dermal mucin (rare)

Seen in many endocrine diseases:
Alopecia in areas of wear (e.g. trunk, elbows)

Potentially bilateral symmetrical trunk alopecia

Alopecic lesions result from hair follicles being predominantly in telogen.

Epidermis and hair follicles exhibit hyperkeratosis

Epidermis is normal to slightly hyperplastic

Question 13

Most common causes of feline hyperthyroidism

Answer 13

Modular hyperplasia

Adenoma

Question 14

Clinical signs of feline hyperthyroidism

Answer 14

• weight loss with ravenous appetite
• PU/PD
• tachycardia
• increased physical activity
• diarrhoea

Question 15

Feline hyperthyroidism and hypertrophic cardiomyopathy

Answer 15

Feline hyperthyroidism potential secondary cause of hypertrophic cardiomyopathy

Approx 10-20% cats with hypertrophic cardiomyopathy have increased coagulability

Can cause thrombus - released into aorta

Abdominal aortic or iliac thrombi-embolism (saddle thrombus) - cause of sudden death

Question 16

Canine thyroid neoplasms

Answer 16

Most are malignant carcinomas
- early pulmonary metastasis
- bone metastases possible later on
- clinical hyperthyroidism infrequently seen even if tumour is functional

Question 17

Feline thyroid neoplasms

Answer 17

Usually benign adenomas
- may be functional

Question 18

Thyroid neoplasia in small mammals

Answer 18

Older guinea pigs get thyroid adenoma and carcinoma

Benign and malignant thyroid neoplasia reported in pygmy hedgehogs