Thyroid Gland Pathology

Question 1

What is an ectopic thyroid?

Answer 1

Thyroid tissue can extend all the way to the diaphragm

Question 2

What are the three endocrine populations present within the thyroid?

Answer 2

1. Thyroid gland follicular epithelium
   * Secrete colloid and iodothyronines
2. Interstitial C-cells/ Parafollicular cells
   * Located in the interstitium, between follicles
   * Secrete calcitonin
3. Parathyroid gland tissue

Question 3

What is the main function of T3/T4?

Answer 3

Maintaining metabolic activity

Question 4

What do thyroid C cells secrete?

Answer 4

They secrete calcitonin in response to hypercalcaemia

Question 5

What is the main function of calcitonin?

Answer 5

Reduces blood calcium by inhibiting osteoclasts
* increased renal PO4 3- and Ca 2+ secretion

polypeptide hormone

opposes the effect of PTH

Question 6

What is the function of the thyroglossal duct?

Answer 6

connects ventral tongue to the thyroid

Question 7

What is primary thyroid hyperplasia?

Answer 7

No formation of the thyroid (rare)

Question 8

What is secondary thyroid hyopoplasia?

Answer 8

Pituitary gland produces less TSH
therefore the thyroid is less stimulated to produce T3/T4
leading to eventual hypothyroidism

Question 9

What is lymphocytic thyroiditis?

Answer 9

autoimmune disease
production of autoantibodies against other antigens

Question 10

What is goiter?

Answer 10

• non neoplastic enlargement of the thyroid gland
• associated with follicular hyperplasia/ hypertrophy

not common in carnivores

Question 11

Where does the thyroid originate?

Answer 11

It originates on the floor of the pharynx- intimately related to the aortic sac

Question 12

What is a thyroglossal duct cyst?

Answer 12

fluctuant mass (potentially full of fluid if it persists)
between the tounge and thyroid

Question 13

Name 4 sequelae for thyroid hypofunction

Answer 13

1. Gain in body weight
2. Integument changes
3. Hypercholesteraemia -> Atherosclerosis
4. Reproductive changes

Question 14

What is lymphocytic thyroiditis?

Answer 14

an autoimmune disease in which thyroid cells are destroyed via cell and antibody-mediated immune responses

Question 15

What can cause goiter?

Answer 15

• Iodine deficiency/ iodide excess
• defective synthesis of thyroid hormones
• Goitrogens -> white clover, linseed meal, sulfonamides

Question 16

What disorders of growth can cause thyroid hyperfunction?

Answer 16

Hyperplasia, adenoma, carcinoma
* common lesion in cats, dogs and horses

Question 17

What usually causes thyroid hypofunction?

Answer 17

idiopathic follicular atrophy

Question 18

Why is clinical hyperthyroidism common in cats?

Answer 18

• T4 is excreted into the bile via glucorinidation
• cats are bad at this
• therefore less T4 is excreted
• Therefore more likely to develop clinical hyperthyroidism

Question 19

What are some sequelae for thyroid hyperfunction

Answer 19

• Hypertension
• Weight loss with increased appetite
• Vomiting
• Physical exam- thyroid slip
• Thyrotoxic Cardiomyopathy (saddle thrombi)

Question 20

What is the most common thyroid disorder in a horse?

Answer 20

Thyroid adenoma

Question 21

What is a common thyroid disorder in ruminants?

Answer 21

C-Cell Neoplasm

Question 22

Explain the steps of iodothyronine synthesis

Answer 22

1. I- enters the follicular cell on the basal side then exits into the colloid
2. Thyroglobulin is synthesised in the RER
3. Thyroglobulin is then secreted into the colloid
4. I- is then oxidised to I2
5. I2 then attaches to tyrosyl residues (forms either monoiodotyrosine or di-iodotyrosine
6. TG is then conjugated to form either T3 or T4

Question 23

Name 5 things that cause thyroid hypofunction

Answer 23

1. Lymphocytic thyroiditis
2. Idiopathic follicular atrophy
3. Bilateral, Non-Functional thyroid tumour
4. Goiter
5. Secondary hypothyroidism

Question 24

What is Hashimotos thyroiditis?

Answer 24

Immune system attacks your thyroid so it cannot produce as many thyroid hormones

Question 25

What is the pathogenesis of lymphocytic thyroiditis?

Answer 25

Production of autoantibodies against TG

Question 26

What is the gross pathology of lymphocytic thyroiditis?

Answer 26

Massive atrophy

Question 27

In what animal species do bilateral non-functional tumours mostly occur in?

Answer 27

Mainly in dogs and horses
over 75% loss of tissue

Question 28

What is the embryology of the thyroid gland?

Answer 28

Originates on the floor of the pharynx- intimately related to the aortic sac
Ectopic thyroid tissue can occur anywhere along this route all the way to the diaphragm

Question 29

What are the main sequelae for thyroid hypofunction?

Answer 29

• Gain in body weight
• Integument changes-> bilaterally symmetric alopecia, hyperkeratosis , hyperpigmentation, myxedema
• Hypercholesterolemia -> atherosclerosis, renal glomerular and corneal lipidosis
• Reproductive changes (libido loss, low sperm count, altered oestrus cycle)

Question 30

What is the pathogenesis of lymphocytic thyroiditis?

Answer 30

• Production of autoantibodies against T
• Gross path= massive atrophy

Question 31

What is goiter not common in?

Answer 31

• Not common in carnivores
• Foetuses and neonates are most severley affected

Question 32

What is the most common thyroid disorder of growth in dogs?

Answer 32

Adenocarcinomas
* often locally invasive and aggressive, resulting in the destruction of remaining normal tissue

Question 33

How are carcinomas formed in cows?

Answer 33

C-Cell stimulation -> hyperplasia -. Adenoma -> Carcinoma