The thyroid gland

Location of the thyroid

anterior neck and spans the C5-T1 vertebrae.

consists of two lobes (left and right), which are connected by a central isthmus anteriorly → butterfly-shape

within the visceral compartment of the neck

What arteries supply the thyroid?

• Superior thyroid artery – arises as the first branch of the external carotid artery.

• Inferior thyroid artery – arises from the thyrocervical trunk (a branch of the subclavian artery)

Venous drainage of the thyroid

superior, middle, and inferior thyroid veins form a venous plexus around the thyroid gland

The superior and middle veins drain into the internal jugular vein

the inferior empties into the brachiocephalic vein

What nerves lie next to the thyroid

superior laryngeal nerve → innervates the larynx

recurrent laryngeal nerve → innervates the larynx

What cells are found in the thyroid ?

follicular cells and parafollicular (C-cells)

The follicular cells are arranged in numerous functional units separated by connective tissue

→ they contain colloid in their lumen → colloid consists of thyroglobulin which is a glycoprotein

The parafollicular cells are found in the connective tissue.

** when the thyroid is inactive, colloid levels are high, the follicles are enlarged and the cells lining them are flattened.

What hormones are released by the thyroid gland?

The secretory products of the thyroid gland are iodothyronines

• thyroxine (T4) (produced in the follicular cells)

• tri-iodothyronine (T3) (produced in the follicular cells)

• calcitonin (produced in the parafollicular cells)

Name some dietary sources of iodine

• fish

• Milk

• Eggs

• Cereal

Synthesis of thyroid hormones

1. Synthesis of thyroglobulin in the ER are packed into vesicles and exocytosie into the lumen of the follicle

2. Active transport of iodide into the epithelial cells against a concentration gradient by a sodium/iodide transporter (NIS) ~ iodine trapping

3. Secretion of iodide into the lumen of the follicle by iodide/chloride transporter (pendrin)

4. Oxidation of iodide to produce iodine by TPO

5. lodination of the side chains of tyrosine residues in thyroglobulin to form MIT and DIT by the enzyme thyroperoxidase ~ organification

6. DIT + MIT → T3 , DIT + DIT → T4 respectively ~ coupling reaction

7. Mature thryroglobulin Containing MIT + DIT, T4 +T3 is endocytosed Back into the follicle cell & can be stored as colloid until it is secreted

8. TSH stimulates peoteolysis of the colloid →T3 + T4 exit via basolaterlateral membrane

Function of thyrocytes

• Collect and transport iodine

• Synthesise thyroglobulin and secrete it into the colloid

• Fix iodine to the thyroglobulin to generate thyroid hormones

• Remove the thyroid hormones from thyroglobulin and secrete them into the circulation

What hormone is the active one and how is the other one converted to make it active?

T3

Deiodinases (D) eg. D1, D2, D3

• conversion of T4 to T3 in peripheral tissues (remove an I- from her outer ring) (D1 - liver, thyroid, kidney + D2- CNS, pituitary gland, placenta)

• conversion of T4 and T3 to inactive metabolites (D3 - CNS , placenta)

What is the Wolff-chaikoff effect?

Iodine metabolism in the thyroid gland can be auto regulated independently of TSH A

• low iodine intake → rate or thyroid synthesis is directly related to available I-

• High iodine intake → suppress TPO → block hormone synthesis

Effects of TSH on the thyroid gland

• increase proteolysis of thyroglobulin

• Increase rate of iodide Trapping

• Increase iodisation of throne to form the thyroid hormones

• Increase size and secretary activity of the thyroid cells

• Increase number of thyroid cells & a change from cuboid to Columnar cells

Effects of thyroid hormones

• Increase O2 consumption

• Increase Na/ K pump

• increase heat production

• Increase cardiac output

• Increase glucose absorption

• Increase glycogenolysis

• Increase gluconeogenesis

• Increase lipolysis

• Increase protein synthesis and degradation

• Bone growth + tooth development