2.1 Anatomy and Blood Supply of Thyroid and Parathyroid Glands

Question 1

What vertebrae levels does the thyroid cover?

Answer 1

C5-T1

Question 2

This nerve lies posterior to the thyroid, and - if damaged during surgery - can cause people to have a hoarse voice

Answer 2

Recurrent laryngeal nerve.

Question 3

The thyroid is ____ to the infrahyoid muscles, and ____ to the common carotid artery/IJVs

Answer 3

• Posterior to infrahyoid “strap” muscles
• Medial to common carotid artery and IJVs

Question 4

What are the three hormones produced by the thyroid? Which is most commonly produced?

Answer 4

• T3
• T4 (Thyroxine)
• Calcitone-in

T4 is most commonly produced; about 80% of thyroid’s output.

Question 5

Describe arterial blood supply to the thyroid

Answer 5

• Superior Thyroid artery (from external carotid; remember SALFOPMS)
• Inferior Thyroid artery (from THYROcervical trunk off subclavian)

Question 6

Describe venous drainage of the thyroid

Answer 6

• Superior thyroid vein (IJV)
• Middle thyroid vein (IJV)
• Inferior thyroid vein (braciocephalic veins)

Question 7

How is the release of calcitonin controlled? Are there any complex feedback loops involved?

Answer 7

• Released in response to increased blood calcium levels.
• Unlike with T3/T4, there are no complex negative feedback loops in the process.

Question 8

True or false: T4 (thyroxine) is the most active form of thyroid hormone.

Answer 8

• False
• In tissues, T4 is converted by deiodinase enzymes into T3, which is the most “activated” form of thyroid hormone

Question 9

What is the main effect of T3? What are some examples of this?

Answer 9

• Main effect is to increase metabolic activity in the body
• This can look like lipolysis, increased heat production, increased energy expenditure in muscle fibres etc.

Question 10

Explain thyroid histology, taking into account every type of hormone the thyroid can release

Answer 10

• Epithelial cells combine to form follicles, which contain thyroglobulin at their centre
• T3 and T4 can be created from thyroglobulin
• C cells (for calcitonin secretion) are parafollicular

Question 11

Why do some people have pyramidal lobes on their thyroid glands? How important is the thyroid during development?

Answer 11

• Some people have pyramidal lobes due to the connections between the tongue and the thyroid during development
• Thyroid hormones are produced during development, and are important for nervous system formation etc.

Question 12

How is the thyroid gland innervated?

Answer 12

• Afferent innervation from vagus nerve
• Autonomic innervation from cervical sympathetic ganglia

Question 13

Histologically, PTH is mainly produced/secreted by ______ cells.

Answer 13

Chief cells.

Question 14

What are the actions of parathyroid hormone? What triggers it?

Answer 14

• Release in response to low calcium levels in blood
• Promotes increased resorption of bone, and increased GI and renal ab/resorption of calcium (these 2 are Vit D dependent)
• Also promotes increased production of active Vitamin D

Question 15

List some common anatomical variations of the parathyroid/thyroid glands

Answer 15

Parathyroid:
- Most commonly one extra/less than normal
- Also can be variation in position (most commonly of inferior ones)

Thyroid:
- No isthmus
- Pyramidal lobe
- Ectopic accessory thyroid tissues