Thyroid Flashcards
What is the weight of the thyroid
25-30g
What is the role of thyroid hormones
Control of metabolism - energy generation and use
Regulation of growth
Multiple roles in development
Describe thyroid hormone synthesis
Produced by follicular thyroid cells
Synthesised from thyroglobulin precursor
Iodine is absorbed from bloodstream and concentrated in follicles
Thyroperoxidase binds to iodine to tyrosine residues in thyroglobin molecules to form MIT + DIT
What are the two thyroid hormones
T3 - Triiodothyronine
T4 - Thyroxine
What is the enzyme involved in thyroid synthesis
Thyroid peroxidase
Describe T3
Biologically active hormone
Produced by mono-deiodination of T4 which is most abundant
Deiodinase (D1,2,3) enzymes present in peripheral tissue
What does MIT + DIT make
T3
What does DIT + DIT make
T4
How does T4 become T3
Deiodination
What % of T4 is free
0.03%
What % of T3 is free
0.3%
What is T3 and T4 bound to
Thyroxine binding globulin
Transthyretin
Albumin
What would hyperthyroidism show
Low serum TSH
High Serum free T4
High serum free T3
What would hypothyroidism show
High serum TSH
Low serum free T4
Low serum free T3
What is the function of the parathyroid gland
Regulate calcium and phosphate levels
Secrete parathyroid hormone in response to
- low calcium
OR
- high phosphate
What are the actions of PTH
Increases calcium reabsorption in renal distal tubule
Increases intestinal calcium absorption (via activation of vitamin D)
Increases calcium release from bone (stimulates osteoclast activity)
Decreases phosphate reabsorption
What is the normal reference range for PTH
1.6 - 6.9 pmol/L
What are the metabolic processes increased by thyroid hormones
Basal metabolic rate
Gluconeogenesis
Glycogenolysis
Protein synthesis
Lipogenesis
Thermogenesis
What is the production of thyroid hormone
Hypothalamus - release TRH
To anterior pituitary - thyrotropes respond to TRH
Secrete TSH
TSH circulates through the blood to the thyroid gland
Production of T3 and T4 (thyroid hormone)
What is the feedback of the thyroid hormone
Too much T3 and T4 - negative feedback to hypothalamus
Low T3 and T4 - feedback to increase hypothalamus
How do T3 and T4 circulate
Via TBG (thyroid binding globule - protein)
What is thyroxine
T4
Describe the production of T3
Can pass into lipid bilayer into cell
5’-deidoinsase - removes iodine off thyroxine
Removal of the iodine molecule on the 5’ of thyroxine (T4)
Produces T3 - active form
Describe the role of T3
T3 binds to a transcription factor/receptor and retinoic acid (RXR) binds
Active transcription factor can move into nucleus
Protein synthesis
Na/k ATPase on the surface - T3 increases expression of these
Due to increase of ATP usage = increase metabolism (metabolic rate)
