Thyroid Hormone

Question 1

The Thyroid gland

Answer 1

Divided into clusters of follicles by connective tissue
Follicles consist of…
- Layers of epithelial cells; synthesise thyroid hormones
- Colloid; protein material, thyroglobulin source of thyroid hormone

Question 2

Thyroid parafollicular C cells

Answer 2

In Follicle lining
- secrete Calcitonin

Question 3

Synthesis of thyroid hormone

Answer 3

• Takes place in the colloid
• Thyroxine and triiodothyronine produced in colloid
• Thyroglobulin produced in follicular cells sent to colloid
• TSH (thyroid stimulating hormone) increases iodine uptake and iodination of thyroglobulin occurs

Question 4

How to tell between active and inactive thyroid follicle

Answer 4

Active has a paler white edge histologically

Question 5

Release of thyroid hormone

Answer 5

• epithelial cells ingest colloid vis endocytosis
• endosomes with colloid fuse with lysosomes, thyroglobulin is digested releasing free thyroid hormones
• Free thyroid hormones diffuse through basal membrane into blood
• Thyroid hormones bind to carrier proteins in blood for transport to target cells

Question 6

Transportation of thyroid hormones

Answer 6

• 99.5% bound to plasma proteins
• Thyroxine binding globulin (TBG) is a glycoprotein synthesized in the liver- HIGH AFFINITY LOW CAPACITY
• Thyroxine binding pre-albumin (TBPA) LOW AFFINITY HIGH CAPACITY
• Albumin (ALB) LOW AFFINITY HIGH CAPACITY

Question 7

REGULATION OF THYROID HORMONES

Answer 7

Hypothalamic-pituitary-thyroid axis
TRH produced in hypothalamus binds to thyrotroph cell in pituitary gland
anterior lobe of pituitary gland is stimulated and produces TSH
- TSH is required for TH release and regulates number and size of follicles
-

Question 8

Physiological function of thyroid hormone

Answer 8

• For normal development of body tissue
• Growth; stimulates growth hormone secretion-promotes synthesis of structural proteins and skeletal growth.
• Activate nuclear transcription of large numbers of genes
• increases overall basal metabolic rate.
• Normal development of the nervous system
• increases blood flow and cardiac output, increase heart rate
• important for function of the testes and ovary

Question 9

Hypothyroidism

Answer 9

• Sub-optimal levels of TH
• Deficiencies typically affects a range of systems – so
  clinical signs are often variable