Thyroid Flashcards
Number of thyroid lobes
2, fused by the isthmus
Blood supply of thyroid gland
Superior and inferior thyroid arteries
Venous drainage via venous plexus (superior, middle, and inferior thyroid veins) into internal jugular vein
Innervation of thyroid gland
Middle/inferior cervical ganglion (sympathetic NS)
Extracellular storage of T3/T4 and thyroglobulin
Colloid
Two required precursors for thyroid synthesis
Thyroglobulin (TG) and iodide
Explain the Wolff-Chaikoff effect
Autoregulation of iodide uptake. If too much dietary iodide is consumed, thyroid hormone production and gland transport is reduced
Iodide deficiency
Mental retardation
Iodide deficiency may lead to this condition
Mental retardation
What is the HPT axis?
Hypothalamus - pituitary - thalamus
Thyroid hormone regulation
HPT axis (paraventricular nucleus (PVN), TRH)
Pathway for T4/T3 synthesis
TRH → TSH → T4/T3
Thyroid hormone regulation
HPT axis
- Hypothalamus (paraventricular nucleus (PVN), TRH)
- Pituitary - thyrotropes (TSH)
Pathway for T4/T3 synthesis
TRH → TSH → T4/T3
Tonic inhibition of thyroid hormone
Somatostatin, dopamine
Iodide trapping → ______
Transport
Transport → _________
Iodination
Iodination → __________
Conjugation
Conjugation → __________
Endocytosis
Endocytosis → __________
Proteolysis
Proteolysis → ___________
Secretion
Common stimulator for all steps in thyroid hormone synthesis
TSH
Thyroid hormone that binds with high affinity to receptor (but low capacity)
T3 (triiodothyronine)
Thyroid hormone that binds with low affinity to receptor
T4 (thyroxine)
Major form of thyroid hormone
T4 (longer half-life, about 7 days)
Major form of thyroid hormone
T4 (longer half-life, about 7 days)
Potency: T4 vs. T3
T3 is about 4x more potent
Inhibitor of thyroid peroxidase
Carbimazole (methimazole). Used as treatment for hyperthyroidism
Formation of T3
One MIT and two DIT residues
Normal iodide uptake %
25%
Hyperthyroidism (% uptake)
> 60%
Hypothyroidism (% uptake)
Graves’ disease and thyroid function
Typically results in hyperthyroidism
Primary source of T3 in circulation
Type I deiodinase
Biological activity of rT3
None
The thyroid hormone “sensor” in the pituitary
Type II deiodinase
The thyroid hormone “sensor” in the pituitary
Type II deiodinase
Primary transport protein with thyroid hormone
TBG (thyroxine-binding globulin, about 70%)
Can TBG reversible release a thyroid hormone to target tissues?
Yes, T4
Thyroid hormone receptor affinity for T4/T3
High affinity, low capacity for T3
Low affinity for T4
Receptor heterodimer that forms after binding of T3
THR & RXR (retinoic acid receptor)
T3 binds this receptor in target cells
THR
Iodine deficiency during development leads to short stature/impaired bone formation, mental retardation, and delayed motor development
Cretinism
This thyroid hormone increases cardiac output
T3
Hyperthyroidism may cause these cardiac anomalies
Arrythmias
Inhibitor of thyroid peroxidase (TP)
Carbimazole (methimazole). Used as treatment for hyperthyroidism
Hyperthyroidism may cause these cardiac anomalies
Arrythmias
Incidence of thyroid cancer
Women:men 3:1
Hyperthyroidism disease, elevated T4/3, autoimmune antibodies, diffuse goiters
Graves’ Disease
Autoimmune destruction of thyroid follicles
Antibodies against TPO (thyroid peroxidase), TG
Hashimoto’s thyroiditis
Emergency life situation, w/ hyperthyroid coupled with severe acute illness
Thyroid storm
Treatment for thyroid storm
PTU (acute)
Carbimazole (methimazole)
Beta blockers (restoration of heart function)