T1 L2: Thyroid hormone physiology Flashcards
What is the arterial blood supply to the thyroid gland?
2 arteries:
- Superior thyroid artery branch of the external carotid artery
- Inferior thyroid artery from the thyrocervical trunk branch of the first part of the subclavian artery
What is the venous drainage of the thyroid gland?
3 veins:
- Superior thyroid vein drains to the internal jugular vein
- Middle thyroid vein drains to the internal jugular vein
- Inferior thyroid vein drains to the innominate veins
What are the innominate veins?
It’s another word for brachiocephalic veins
What is the lymphatic drainage of the thyroid gland?
To the pretracheal and prelaryngeal then to the deep cervical, supraclavicular and medistinal lymph nodes
Describe the cellular composition of the thyroid gland
Composed of follicles each consisting of a monolayer of epithelial cells enclosing a large core of viscous, homogenous colloid
What is the colloid for?
It acts as a reservoir for the thyroid hormone
What is Tri-iodothyronine also known as?
T3
What is Tetraiodothyronine also known as?
T4
What are the 3 primary hormones secreted by the Thyroid gland?
T3, T4, and calcitonin
How is T3 produced?
Iodine and Tyrosine form Monoiodotyrosine (MIT) and Diiodothyronine (DIT).
DIT plus MIT create T3
Alternatively, T4 can be broken down by iodothyronine deiodinase in tissues to create T4
How is T4 produced?
Iodine and Tyrosine form Monoiodotyrosine (MIT) and Diiodothyronine (DIT).
DIT plus DIT create T4
What % of thyroid hormone leaves the thyroid gland in the form of T4 (thyroxine)?
95%
What degrades MIT and DIT and why?
Halogenases to free the iodine, which is then re-utilised by combination with thyroglobulin
What is reverse T3 (rT3)?
The biologically inactive form of triiodothyronine (T3)
T3 and rT3 have the same structure but is just arranged differently
Where are receptors for thyroid hormones found?
They’re intracellular (in the nucleus) just like for other steroid hormones
What effect do thyroid hormones have on the cell when they bind to receptors?
They influence gene expression and thus protein synthesis
Which hormones control rate at which chemicals are taken up by cells in the body?
Thyroid hormones
What is the main effect of thyroid hormones?
To increase basal metabolic rate
What are 3 ways to increase the basal metabolic rate?
- Increase carbohydrate metabolism
- Increase synthesis, metabolism, and degradation of lipids
- Increase protein synthesis
How can you accurately measure the basal metabolic rate at the bedside?
By measuring the basal heart rate
Which cells produce Calcitonin?
Parafollicular ‘C’ cells
What is TRH?
Thyrotropin releasing hormone
What is TSH?
Thyroid stimulating hormone
Which is more biologically active, T4 or T3?
T3
It’s about 40x more active than T4
What is the plasma half-life of T3 and T4?
T3 - 1 day
T4 - 6-8 days
Which 3 proteins does Thyroxine bind to while circulating in the blood?
Thyroxine binding globin
Thyroxine binding Albumin
Thyroxine binding Prealbumin
What inhibits the release of TSH directly and how?
T3 and T4 inhibit TSH by supressing the release of TRH
It all creates a cycle with T3 and T4 creating a negative feedback loop for themselves
What do hypothalamic releasing hormones do?
They control the secretion of ganadotropes, somatotropes, lactotropes, corticotropes, and thyrotropes
How does the body get rid of T3 and T4?
T3 and T4 is conjugated and excreted as bile
What is a Derbyshire neck?
A goitre formerly endemic to Derbyshire because the area is deficient in iodine
What makes an area deficient in Iodine?
Being far away from the sea because iodine is abundant in seaweed and the fish
Why can the body become deficient in iodine during pregnancy?
Because the requirements for iodine go up
How can Hyperthyroidism be treated?
By reducing thyroid hormones with drugs or removing the thyroid by surgery or radioactive iodine
How can hypothyroidism be treated?
Replacement therapy
What is Levothyroxine used to treat? How does it work and how can it be administered?
Hypothyroidism by supressing TSH secretion. Can be given by mouth or injection
What are some adverse effects of Levothyroxine?
Palpitations Arrhythmias Diarrhoea Insomnia Tremor Weight loss
What is the drug Carbimazole used to treat? How does it work?
Treats hyperthyroidism
It’s converted into Methimazole upon absorption which prevents peroxidase iodinating the tyrosine residues on Thyroglobulin. This reduces the levels of T3 and T4
What are some adverse drug effects of Carbimazole?
Rashes and Pruritus are common but can be treated with antihistamines. Can cause Neutropenia and agranulocytosis. It’s tetragenic
What is Pruritus?
Itchy skin
What is Neutropenia?
Few to no white blood cells
What is Agranulocytosis?
Few to no granulocytes
What is the drug Propylthiouracil used to treat? How does it work?
Treatment for Hyperthyroidism for pregnancy
It inhibits thyroperoxidases which normally act in thyroid hormone synthesis
What are some adverse effects of Propylthiouracil?
Rashes and Prutirtus that can be treated with antihistamines. Can cause agranulocytosis and risk or serious liver damage including liver failure and death. Is not tetragenic
What effect does iodine have on radiation?
It blocks absorption of radiation
