Session 10 Flashcards
When is the thyroid gland visible and palpable?
- When it is enlarged (goitre)
Where is the thyroid gland located?
- In the neck in the front of the lower larynx and upper trachea
What else is near to the thyroid gland?
- 2 nerves (recurrent laryngeal and external branch of the superior laryngeal)
- Highly vascularised with 3 arteries and veins supplying and draining it (superior, middle and inferior thyroid arteries and veins)
What is the structure of the thyroid gland?
- Butterfly shape
- 2 lateral lobes connected by a central isthmus
- Usually 2-3 cm and 15-20g
What are the main cells types of the thyroid gland?
- Follicular (arranged in follicles separated by connective tissue)
- Parafollicular (C-cells) (found in the connective tissue)
What hormones are produced by the follicular cells of the the thyroid gland?
- Throxine (T4)
- Tri-idothyronine (T3)
- Small molecules derived from amino acid tyrosine with the addition of atoms of iodine
What hormone is produced by the parafollicular cells of the thyroid gland?
- Calcitonin
- Polypeptide hormone
How are T3 and T4 synthesised in the thyroid follicles?
- Transport of iodine into epithelial cells against a concentration gradient
- Synthesis of a tyrosine rich protein (thyroglobulin) in the epithelial cells
- Secretion by exocytosis of thyroglobulin into the lumen of the follicle
- Oxidation of iodine to produce an iodinating species
- Iondination of the side chains of tyrosine residues in the thyroglobulin to form MIRA (mono-iodotyrosine) and DIT (di-iodotyrosine)
- Coupling of DIT with MIT/DIT to form T3/T4 respectively within the thyroglobulin
- T3 and T4 residues are produced in the ratio of 1:10
How are T3 and T4 stored?
- Extracellularly in the lumen of the follicles as part of thyroglobulin molecules
How are T3 and T4 secreted?
- Thyroglobulin is take into epithelial cells by Endocytosis from the lumen of the follicles
- Thyroglobulin undergoes proteolytic cleavage to release T3 and T4
- T3 and T4 diffuse from the epithelial cells into the circulation
What controls T3 and T4 synthesis and secretion?
- Hypothalamus: releases Thyrotrophin-releasing hormone (TRH)
- Anterior pituitary gland: releases Thyroid Stimulating Hormone (stimulated by TRH) and affects follicular cells of thyroid gland
What is TRH secretion influenced by?
- Circulating levels of T3 and T4 (negative feedback)
- Stress (increases release)
- Temperature (fall increases release)
What is the structure of TSH?
- Glycoprotein consisting of 2 non-covalently linked subunits
When is TSH a released?
- Released in low-amplitude pulses
- Diurnal rhythm: higher levels at night, lower levels in the early morning
What are the actions of TSH on follicle cells and how?
- Interacts with cell surface receptors
- Stimulates synthesis and secretion of T3 and T4
- Has trophic effects on the cell -> increased vascularity, size and number of cells
- Can result in an enlarged thyroid (goitre) that may or may not be active
How are T3 and T4 transported?
- Are hydrophobic
- Transported in the blood bound to proteins (thyronine binding globulin, pre-albumin and albumin)
- Only a small amount is free in solution
- Free hormone is biologically active
- T3 has a shorter half-life and a higher free percentage than T4 as T3 has a slightly lower affinity for transport proteins