7. Thyroid hormone physiology Flashcards
Are thyroid hormones essential for life?
- not essential for life
- essential for normal development
- essential for physical and mental wellbeing
Where is the thyroid gland located?
- neck
- around larynx and trachea
- 4 parathyroid glands
Describe anatomy of thyroid gland
- 2 lobes
- isthmus in middle
What is thyroid gland composed of?
- thyroid gland is composed of follicles
- follicles made of a monolayer of epithelial cells enclosing a large core of viscous, homogeneous colloid
- Colloid acts as a reservoir of thyroid hormone
What does colloid act as?
reservoir of thyroid hormones
What are the primary molecules secreted by the thyroid gland?
- tri-iodothyronine (T3)
- tetraiodothyronine(T4, thyroxine)
- calcitonin
What is calcitonin responsible for?
calcium homeostasis, and is independent of other thyroid hormones
What is the thyroid hormone pathway?
iodine & tyrosine-> monoiodine rosine (MIT) and dioiodotyrosine (DIT)
DIT + DIT -> tetraiodothyronine (T4)
MIT+ DIT -> triiodothyronine (T3)
In tissues, T4 made into T3 by iodothyronine deiodinase
How is T4 made into T3
by iodothyronine deiodinase in tissues
How is iodine freed from MIT and DIT?
- MIT and DIT are rapidly degraded by halogenases to free the iodide
- iodide reutilised by combining with thyroglobulin
How do T3 and T4 get to target tissues?
- T3 and T4 leave the follicular cells and enter bloodstream for distribution to target tissues
- 95% of thyroid hormone leaving thyroid gland is in the form of T4 (thyroxine)
What is the difference in effect between T3 and T4?
- deiodinase enzymes convert the T4 to T3 (80%) or reverse T3(20%) in target tissues
- T3 has a biological activity approx. 40 times greater than that of T4, whilst reverse T3 is biologically inactive
- up to 90% of the biologically active thyroid hormone within the cell is in form of T3
- Plasma half-life of T4 is 6-8 days whilst T3’s is one day
What does iodine deficiency cause?
higher TSH
Derbyshire neck
How does thyroid hormone travel around the body?
- insoluble in water, transported in blood bound to plasma proteins
- over 99% of circulating thyroid hormones protein bound
- most of T4 (75%) is bound to thyronine-binding globulin (TBG)
- 15-20% of T4 bound to thyroxine-binding prealbumin (TBPA)
- 5% bound to albumin
Describe thyroid hormone receptors
- Intracellular, more specifically nuclear
- effect of interaction of thyroid hormones with receptors is to influence gene transcription and protein synthesis
What is the main effect of thyroid hormones?
to increase basal metabolic rate:
- increase carbohydrate metabolism
-increase in synthesis, mobilisation and degradation of lipids
-increased protein synthesis
thyroid hormones essential for normal development of the CNS, especially myelination of nerve fibres
What is the primary mechanism of thyroid hormone?
Increase number and size of mitochondria and increased activity of metabolically important enzymes
All aspects of carbohydrate metabolism are increased:
- glycogenesis and glucose uptake by muscle cells and adipose cells
- potentiation of effects of insulin
- potentiation of effects of catecholamines
- increase in glucose absorption by gastrointestinal tract
thyroid hormones increase basal metabolic rate and therefore oxygen consumption in nearly every organ except:
- brain
- uterus
- testes
- spleen
- thyroid gland
- anterior pituitary gland
What are some anti-thyroid drugs and how do they work?
carbimazole
methimazole
inhibition of synthesis of thyroid hormones by preventing the incorporation of the iodide into the thyroglobulin
What does propylthiouricil do?
propylthiouricil prevents peripheral conversion of T4 to T3
How does potassium perchlorate work?
acts by competing with iodide for the active iodide uptake pump
What can radioactive (131I) iodine do?
- alternative to surgery
- selectively concentrated in thyroid gland where it causes tissue damage and therefore reduced thyroid hormone secretion
What can prophylactic administration of iodine cause?
- prophylactic administration of iodine through either injection or incorporation into table salt/flour has reduced incidence of endemic goitre worldwide
- iodine carries risk of Jod-Basediw phemonon in which iodine administration precipitates hyperthyroidism
What drugs can induce goitre?
- lithium, used in treatment of bipolar depression
- iodides contained in vitamin preparations and some cough remedies
- these ions are selectively concentrated within the thyroid gland where they interfere with iodide incorporation and hormone release
