Session 10 ILOs - The Thyroid Gland and thyroid disorders

Question 1

Describe the location and structure of the thyroid gland

Answer 1

Location:

• In the neck, in front of the lower larynx and upper trachea
• Only visible or palpable when enlarged (goitre)
• Two nerves lie in close proximity to the gland (1) recurrent laryngeal (2) external branch of the superior laryngeal - so are at risk during thyroid surgery

Structure:

• Butterfly shape with two lateral lobes joined by a central isthmus
• Usually 2-3cm across and is one of the largest endocrine glands in the body
• Two major cell types are found in the gland
  1. Follicular cells - arranged in numerous functional units called follicles separated by connective tissue
  2. Parafollicular (C cells) - found in the connective tissue

Question 2

Describe the chemical structure of the thyroid hormones and the mechanisms of their production, storage and secretion

Answer 2

Thyroid gland produces 3 hormones

1. Thyroxine (T4) - produced in the follicular cells
2. Triiodothyronine (T3) - produced in the follicular cells
3. Calcitonin produced in the parafollicular cells.

T3 & T4 are derived from Tyrosine with the addition of atoms of Iodine

Production - occurs in the thyroid follicles:

• Transport of iodide into the epithelial cells (against conc gradient)
• Synthesis of thyroglobulin in the epithelial cells from amino acids
• Thyroglobulin is exocytosed into the lumen of the follicle
• Iodide is oxidised to iodine
• Iodination of thyroglobulin (specifically the side chains of the tyrosine residues) to form MIT (mono-iodotyrosine) and DIT (di-iodotyrosine)
• Coupling of DIT with MIT or DIT with DIT to form T3 & T4 respectively

Storage:

• T3 & T4 are stored in the lumen of the follicles (extracellular) as part of the thyroglobulin molecules
• The amounts normally stored are considerable and would last for several months at normal rates

Secretion:

Question 3

Describe how the activity of the thyroid gland is controlled

Answer 3

• Thyroglobulin is taken into the epithelial cells from the lumen of the follicles by the process of endocytosis
• Here proteolytic cleavage of the thyroglobulin occurs to release T3 & T4 and these diffuse from the epithelial cells into the circulation

Question 4

Describe the effects of thyroid hormones on cells and the body as a whole

Answer 4

Main general effects (interlinked):

1. Metabolism
2. Growth and development
   - Increase basal metabolic rate and heat production (increase number and size of mitochondria and increase synthesis of respiratory chain enzymes)
   - Stimulation of metabolic pathways (generally catabolic pathways increase lipid and carbohydrate metabolism)
   - Sympathomimetic effects (increase response to catecholamines)

Tissue specific effects:

• Increases heart’s responsiveness to catecholamines (increase CO and peripheral vasodilation)
• Increases myelination of nerves and development of neurones

Question 5

Describe the consequences of over- and under-secretion of thyroid hormones

Answer 5

Underactive - hypothyroidism:
- Obesity, leathery, intolerance to cold, bradycardia, alopecia, dry skin, constipation
Children = Cretinism
Adults = Myxoedema
Also Hashimoto’s disease - autoimmune disease in which the body destroys thyroid follicles

Overactive - hypothyroidism:
- Weight loss, irritability, heat intolerance, tachycardia, increased bowel movements, fatigue or weakness
Also Graves’ disease - autoimmune disease in which the body produces TSI (thyroid stimulating immunoglobulin)