The Thyroid Hormone Flashcards
Where are hormones produced?
Endocrine glands
Where are hormones secreted into?
Bloodstream
Where are hormones aiming to reach?
Target cells
Act on receptors
What kinds of effects can hormones have?
Rapid- Adrenaline (Epinephrine, target the heart)- increase in heart rate
Slow in onset and longer lasting- Androgens- build up of muscle tissue
Give two examples of hormones?
Thyroid hormones
Adrenal gland hormones
What does the hypothalamus act on?
The anterior pituitary
What effect does the hypothalamus on the anterior pituitary gland?
Leads to increase or decrease in release of hormone?
Where us the thyroid gland located and what’s it shape?
Located in the neck
Butterfly shaped
What does the hypothalamus produce?
Thyrotrophin Releasing Hormone (TRH)- a tripeptide
What does the anterior pituitary gland release in response to TRH?
Produces Thyroid Stimulating Hormone (TSH) or thyrotrophin a glycoprotein
What thyroid hormones does the thyroid gland produce?
Tri-iodothyronine- T3
Tetra-iodothyronine- Thyroxine- T4
Where are thyroid hormones synthesised and stored?
In the thyroid gland
Where and when are thyroid hormones release?
Released when needed
Circulate in blood to their target tissues
What do thyroid hormones control?
How much TRH and TSH is released
What is the thyroid functional unit?
The follicle
What is a follicle?
Single layer of epithelial cells around a cavity (follicle lumen) which is filled with colloid containing thyroglobulin
What is thyroglobulin?
A large glycoprotein (contains tyrosine residues) is synthesised, glycosylated and then secreted into the lumen of the follicle, where iodination of the tyrosine residues occurs
What are the main steps of synthesis, storage and secretion of thyroid hormones?
Uptake of plasma iodide (iodide uptake, the main control point for hormone synthesis, is stimulated by TSH) by follicle cells
Oxidation of iodide to iodine (by peroxidase)
Iodine transferred to tyrosine residues in Thyroglobulin to give mono-iodinated and di-iodinated tyrosine (MIT and DIT)
Secretion of thyroid hormone
What are the 8 steps of the synthesis of a thyroid hormone?
- Active uptake of iodide (I-)
- Iodide may be discharged
- Iodide uptake stimulated by TSH
- Oxidation of iodide
- Active transport of iodine
- Formation of MIT and DIT
- Uptake of the thyroglobulin
- About 1% of stored colloid is removed each day. When the gland is very active this may rise to nearly 100% and colloid stores are depleted
Where are T3 and T3 synthesised?
The follicles of the thyroid gland
What is the shape of a follicle?
Sphere
Filled with glycoprotein, thyroglobulin which contains many tyrosine residues
What does TSH act on?
Receptors on the membrane of the thyroid follicle cells through the mechanism that involves cAMP
What does TSH acting on the membrane activate?
Hormone synthesis
What do iodinated tyrosines combine to form?
Tri-iodothyronine (T3) and tetra-iodothyronine (T4)
Where are T3 and T4 stored?
On thyroglobulin
What happens when blood levels of T3 and T4 are taken up by endocytosis into the follicle cells?
The endocytotic vesicles fuse with lysosomes and proteolytic enzymes act on thyroglobulin, releasing T3 and T4 enter the blood stream
How do T3 and T4 move in the blood?
> 95% bound to plasma proteins
What are the nuclear receptors?
Intracellular
What is T4 converted to inside the cell?
T3 which acts on the receptor
What does the receptor associate with?
DNA
What does T3 acting on the receptor lead to?
mRNA and protein synthesis
What are thyroid hormones involved in?
Increasing the basal metabolic rate
Increased heat production (oxidative metabolism)
Increase glucose and amino acid uptake into cells
Mitochondria- increase number and size
mRNA and mRNA polymerase activity
Protein synthesis
All achieved through regulation of nuclear receptor activity
What is hypothyroidism for Hashimoto’s Thyroidtis?
Lack of a hormone
What does hypothyroidism in infancy for Hashimoto’s Thyroidtis show symptoms of?
Mental retardation
Dwarfism
What are symptoms of hypothyroidism in Hashimoto’s Thyroidtis?
Depressed metabolic rate Depressed appetite Depressed Cardiac ouput Tired Lethargic
How is hypothyroidism treated?
Giving T4- Thyroxine
What are the symptoms of hypothyroidism in Grave’s disease?
Enlarged thyroid (goitre) Protruding eyes (exophthalmos)
How is Grave’s Disease caused?
Antibody that mimics the effects of TSH is produced
Leads to excess stimulation of the thyroid gland
Causes increased release of T3 and T4
Excess of hormone
Increased metabolic rate, appetite
Sympathetic activity increased
Nervous and motor activity increased
How is Grave’s disease treated?
Surgery
Irradiation with iodine 131
Iodides
Drugs which inhibit hormone synthesis
What is Thioureylenes?
E.g. Propylthiouracil; Methimazole; Carbismazole
Inhibit peroxidase conversion of iodide to iodine
Reduces iodination of tyrosine residues in thyroglobulin
Coupling of the iodinated tyrosine to form T3 and T4
Conversion of T4 to the active form T3
What do thyroid hormones promote the synthesis of?
Adrenoceptors
How can hyperthyroidism be treated?
Treated with beta adrenoceptor antagonists
