E3 - Thyroid Gland Flashcards
Thyroid Gland Histology
Made of follicles
– Single layer of follicular epithelial cells surrounding a lumen full of proteinaceous colloid.
– Synthesize thyroid hormone
Thyroid hormone synthesis
- Iodine entry and concentration in thyroid follicular cells
- Thyroid hormone synthesis/storage in the colloid
- Release of thyroid hormone into the blood
___ regulates NIS gene expression, prolongs NIS half- life and mobilizes NIS to cell membrane
TSH
TSH regulates rate of organification and coupling by stimulating ___ gene expression
TPO
Thyroid hormone secretion into blood
- Endocytosis of colloid
- Proteolysis releases T3/4
- Deiodination releases I from MIT/DIT
NIS
(Iodide transporter)
Cotransports Na+ and I- across basolateral membrane into cell
Thyroid Peroxidase (TPO)
Catalyzes formation of thyroid hormone within thyroglobulin protein sequence through oxidation and organification of iodide and coupling of iodinated tyrosine residues within the protein
Thyroglobulin
A glycoprotein that constitutes ~50% of thyroid cell protein
Thyroid hormones (primarilyT3) are important for early growth and development.
-CNS: cognition and mood
-Sexual development: timing of puberty, menses
-Linear growth (modulate GH)
Hypothyroidism Tx
Synthetic thyroid hormone replacement
– Levothyroxine (synthetic T4)
Hyperthyroidism Tx
Medication to decrease thyroid hormone synthesis
– Methimazole, PTU (Propylthiouracil): Inhibit TPO
- ___ of T4 is secreted by the thyroid gland ~ 100nmol daily
- ___ of T3 is secreted by the thyroid gland ~ 5nmol daily
- ___ of T3 is formed from peripheral conversion of T4 to T3
- 100%
- 20%
- 80%
5’ deoidinase in peripheral cells is inhibited by:
-Fasting ≥ 2 days
-Severe illness
-Glucocorticoids
-Certain drugs
Many cells express a 5’ de-iodinase that ____ the 5’ I of T4 to convert it to T3
Removes
Thyroid hormones are ___ in water, and are ____ bound to plasma proteins to facilitate their distribution through the blood
-poorly soluble
-reversibly
Most thyroid hormones are bound to ___
TBG
– T3 half-life is ____
– T4 half-life is ____
-1 day
-1 week
Changing TBG levels changes ____ T4 and T3 concentrations in blood but ___ levels remain stable
-Changes total
-Free remains same
___ has 5-10x more biological activity than ___ on the TR
T3 has 5-10x more biological activity than T4 on the TR
Thyroid hormone receptor (TR) is a ____ receptor
Nuclear
T3 binds to the ligand-binding
domain, activating transcription factors that regulate gene expression
Iodine sources for thyroid hormones
- Dietary Iodine
- Plasma Iodide
- Pharm (meds/CT contrast)
The thyroid gland ____ hormone production in response to prevent thyroid hormone excess or deficiency due to acute changes iodine availability
Autoregulates
“Wolff-Chaikoff” effect