Thyroid Flashcards
What hormones are produced by the thyroid gland?
T3 and T4
Composed of 2 tyrosines linked together w/ e or 4 iodines
What is thyroglobulin?
A protein on which thyroid hormones are synthesised
Contains many tyrosines
Thyroid hormones are synthesised by iodisation then coupling
What is thyroid peroxidase?
An enzyme involved in synthesis of thyroid hormones
What are the 3 reactions carried out by thyroid peroxidase?
Oxidation; iodide to iodine
Addition of iodine to tyrosine acceptor residues on thyroglobulin
Coupling of MIT or DIT
Why is most fo T4 converted to T3?
T3 has a higher biological activity
Where is T4 converted to T3?
Liver and kidneys
What protein are thyroid hormones bound to in circulation?
Thyroxine binding globulin
What hormone stimulates release of thyroid hormones?
TSH (from ant pituitary)
What actions are stimulated by TSH?
Iodine uptake Iodine oxidation Thyroglobulin synthesis Thyroglobulin oxidation Cell metabolism and growth Colloid pinocytosis
What G proteins does TSH act on?
Gαs
Gαq
How do thyroid hormones increase BMR?
Increase number and size of mitochondria
Stimulate synthesis of enzymes in respiratory chain
What is the effect of thyroid hormones on metabolic pathways?
Lipid metabolism;
Stimulate lipolysis and β-oxidation
Carbohydrate metabolism;
Stimulate insulin-dependent entry of glucose
Increase gluconeogenesis and glycogenolysis
What is the sympathomimetic effect of thyroid hormones?
Increase adrenoceptor number to increase response to cathecholamines
What is the effect of thyroid hormones on the cardiovascular system?
Increase response to catecholamines
Increase CO
Increase peripheral vasodilation => carry body heat to surface
What is the effect of thyroid hormones on the nervous system?
Essential for adult function and development
Increase myelination and development of neurones
How do thyroid hormones exert their effects?
They function as hormone-activated transcription factors
Modulate gene transcription
Binds to thyroid hormones on DNA => relieves repression of gene transcription => expression of DNA
What are some examples of thyroid activated genes?
PEPCK
Na/K ATPase
Ca-ATPase
What is a goitre?
Enlargement of the thyroid gland due to overstimulation
Can accompany hypo- or hyperthyroidism
What is hypothyroidism?
Low levels of thyroid hormones
What are some causes of hypothyroidism?
Failure of thyroid gland TSH/TRH deficiency Autoimmunity (Hashimoto’s) Congenital Post surgery (thyroidectomy)
What are some symptoms of hypothyroidism?
Fatigue Weight gain Cold intolerance Goitre Constipation Menorrhagia
What is Hashimoto’s?
Autoimmune condition against thyroid follicles resulting in hypothyroidism
What is hyperthyroidism?
Excess thyroid hormones
What are some causes of hyperthyroidism?
Autoimmune - Graves’ Toxic multinodular goitre Thyroid carcinoma Drugs eg amiodarone Ectopic thyroid tissue
What are some Sx of hyperthyroidism?
Weight loss Irritability Heat intolerance Fatigue Hyper-reflexive Breathlessness Increased bowel movement
What is Graves’ disease?
Autoimmune condition => hyperthyroidism
Production of thyroid stimulating immunoglobulin (TSI)
Continuous stimulation of thyroid hormone outside of -ve feedback
What is the medical treatment of hyperthyroidism?
Thionamides - carbimazole, propylthiouracil
Use β-blockers to control Sx until thyroid function is normal
What is a side effect of thionamides?
Agranulocytosis
Do FBC if pt develops unexplained fever or sore throat, exclude pancytopaenia
What is definitive treatment of hyperthyroidism?
Radioactive iodine
Thyroidectomy
What are some signs of hypothyroidism?
Bradycardia
Ataxia
Dry hair and skin
Slow reflexes
What is the treatment for hypothyroidism?
Levothyroxine
50-100mcg /day
Elderly or ischaemic heart disease - 25mcg
