Physiology 5 - Thyroid Flashcards
List the cell types of the thyroid?
- C (clear) cells (Parafollicular cells)
- Follicular Cells
What do C cells do?
Release calcitonin
Describe the structure of follicular cells?
Cuboidal cells that make the wall of thyroid follicles, containing colloid
What do Follicular cells do?
They produce Thyroglobulin (precursor to thyroid hormones) and the enzymes that make them.
They also concentrate iodide into the colloid for use in making thyroid hormones
How do follicular cells concentrate iodide in the colloid?
Recieve it from the plasma through a Na+/I- symporter using Na+ conc. gradient.
Pump it through to colloid through Pendrin Transporters
What is thyroglobulin?
Glycoprotein containing many tyrosine residues used to make thyroid hormones
What enzyme catalyses thyroid hormone synthesis?
Thyroid Peroxidase
What thyroid hormones are made in the colloid?
T3 - Triiodothyronine
T4 - Thyroxine
Both are amine hormones made from tyrosine, hence they’re lipophilic
Describe the process of Thyroid hormone synthesis in the colloid?
1) iodide oxidised to iodine
2) Iodine added to Tyrosine Residues in thyroglobulin
(1 iodine = MIT, 2 iodine = DIT)
3) MIT + DIT = T3 or DIT + DIT = T4
What do MIT and DIT stand for?
Monoiodotyrosine
Diiodotyrosine
Tyrosine-iodine complexes which can be combined to form thyroid hormones
What happens to transport the thyroid hormones out the colloid and cleave them into active forms?
Thyroid Stimulating Hormone triggers:
1) Endocytosis of colloid
2) Proteolytic enzymes then cut thyroglobulin to release THs
3) T3/4 diffuse through to plasma
4) most bind to plasma proteins
What plasma proteins bind T3/4?
Mainly Thyroxine-binding Globulin (TBG)
Summary of TH production:
Iodine combined with tyrosine residues by Tyrosine Peroxidase in the Colloid.
TSH triggers cleaving and release of THs into plasma
Whats the two main differences between T3 & T4?
T4 has a greater affinity for TBG, hence it binds better so has a longer half life (6 vs 1 day).
T3 has a greater affinity for Thyroid Hormone Receptors so is more physiologically active
In what form is most plasma thyroid hormone?
Protein bound T4
What happens to T4 prior to action on Thyroid hormone receptors?
Its deiodinated to T3 by deiodinase enzymes.
Either in plasma or target cells.
The degree of deiodination can change based on need for Active THs
What factors regulate Thyroid Hormones?
- Free Hormone (-ve feedback)
- Cold/Exercise/Pregnancy
- Glucocorticoids
- Somatostatin
How do Free Thyroid Hormones regulate themselves?
By -ve feedback
Free T3 or T4 inhibits TSH release from adenophysis and TRH from hypothalamus
How does cold, exercise and pregnancy affect thyroid hormones?
They all stimulate TRH release from the hypothalamus
How do glucocorticoids affect thyroid hormones?
Inhibit TSH release & T4 deiodination
How does somatostatin affect thyroid hormones?
Inhibits TSH release
Where are T3 receptors found?
Nuclei
What does T3 do?
- Raises metabolic rate
- Promotes Thermogenesis
- Promotes gluconeogenesis
- Promotes Proteolysis
- Promotes Lipolysis
- Stimulates GH receptor expression (permissive)
- Foetal Brain development
- Sensitizes Beta receptors to epinephrine (Permissive)
