Thyroid gland and thyroid disorders Flashcards
Where is the thyroid gland located?
In the neck in front of the lower pharynx and upper trachea.
What is the structure of the thyroid gland?
A butterfly shape with two lateral lobes joined by a central isthmus. It is one of the largest endocrine glands in the body.
What is the chemical structure of thyroid hormones?
T3 and T4 are small molecules derived from the amino acid tyrosine with the addition of atoms of iodine.
How are thyroid hormones produced?
- Transport of iodide into the epithelial cells against a concentration gradient.
- Synthesis of a tyrosine rich protein (thyroglobulin) in the epithelial cells
- Secretion (exocytosis) of thyroglobulin into the lumen of the follicle
- Oxidation of iodide to produce an iodinating species
- Iodination of the side chains of tyrosine residues in thyroglobulin to form MIT (mono-iodotyrosine) and DIT (di-iodityrosine)
- Coupling of DIT with MIT or DIT to form T3 and T4 respectively within the thyroglobulin.
How are thyroid hormones stored?
T3 and T4 are stored extracellularly in the lumen of follicles as part of thyroglobulin molecules. Considerable amounts are stored, they would last several months at normal rates of secretion.
How are thyroid hormones secreted?
Thyroglobulin is endocytosed into the epithelial cells. Proteolytic cleavage of the thyroglobulin occurs to release T3 and T4 which diffuse from the epithelial cells into the circulation
How is the activity of the thyroid gland controlled?
Hypothalamus: Thyrotropin-releasing hormone (TRH)
A. pituitary: Thyroid stimulating hormone (TSH)
Negative feedback: T3 and T4
What are the metabolic effects of thyroid hormones on cells and the body as a whole?
Increase the metabolic rate of many tissues. They are generally catabolic and lead to an increase in BMR, heat production and oxygen consumption.
Stimulate glucose uptake and metabolism
Stimulate mobilisation and oxidation of fatty acids
Stimulate protein metabolism
What is a goitre?
When a thyroid gland becomes enlarged and is visible or palpable, due to overstimulation. This can accompany either hypothyroidism or hyperthyoidism, but not necessarily present in either.
What is at high risk of getting damaged during thyroid surgery?
Two nerves which lie in close proximity to the gland (the recurrent laryngeal and external branch of the superior laryngeal). These nerves innervate the larynx and are involved in speech.
What are the names of the three arteries and three veins supplying the thyroid?
Superior, middle and inferior thyroid arteries and veins
What are the two major cell types found in the thyroid gland?
Follicular and parafollicular (C-cells)
What is the name of the functional unit that follicular cells arrange themselves in?
Follicles
What separates each follicle from each other?
Connective tissue
Where are the parafollicular cells located?
In the connective tissue
Describe the structure of the follicle
Follicles are spherical and are lined with epithelial (follicular) cells surrounding a central space (lumen) containing colloid which is rich in the protein thyroglobulin
What are the three hormones that the thyroid produces?
T3 - triiodothyonine
T4 - thyroxine
calcitonin (only one produced in parafollicular cells)
What is the chemical structure of calcitonin?
Polypeptide hormone
T3 and T4 are produced in what ratio?
Approximately 1:10
The tri-peptide TRH secretion is increased by what factors?
Increased stress and fall in temperature
What is the name of the cells that produce TSH in the anterior pituitary?
Thyrotropes
What is the target cells of TSH?
Follicilar cells in the thyroid
Which type of hormone is TSH?
Glycoprotein with two non-covalently linked subunits (alpha- and beta-)
What rhythm is TSH released in?
Diurnal - higher levels during night which decrease in early hours of the morning
What are the actions of TSH?
It interacts with receptors on the surface of follicular cells and stimulates all aspects of the synthesis and secretion of T3 and T4,
It also has trophic effects on the gland which increases its vascularity, size and number of follicle cells. Which can result in an enlarged thyroid (goitre).
How is T3 and T4 transported in the blood?
They are hydrophobic and the majority are transported bound to proteins (thyronine binding globulin (TBG), pre-albumin and albumin). Only a small amount (
Which thyroid hormone (T3 of T4) has a longer half-life in blood and why?
T4 has a longer half-life (8 days vs 2 days) because it has a higher afinity for the transport proteins than T3. Therefore a greater % of T3 protein is free
What affect does oestrogen produced during pregnancy have on T3 and T4?
Oestrogen increases the synthesis of TBG during pregnancy -> decrease in free T3 and T4 in circulation (more is bound). This reduced the negative feedback of T3 and T4, so more TRH and TSH are produced and hence more T3 and T4. Overall this returns the level of free T3 and T4 in circulation, but increases the total amount of T3 and T4 in the blood