Thyroid gland anatomy
- 2 lateral lobes
- medial isthmus
- follicle cells create spaces called follicles which are filled with colloid
- parafollicular cells
Colloid
- describe
- amber, sticky
- thyroglobulin and iodine (makeup of TH)
Follicle cells
- describe
- squamous and cuboidal thyrocytes
- produce thyroglobulin and secrete it into the follicle/follicular space
Parafollicular cells
- describe
- secrete calcitonin
- fewer in number compared to follicle cells
Two hormones produced and secreted by the thyroid gland
- thyroid hormone
2. calcitonin
Thyroid hormone
- functions
- cellular metabolism
- regulate lipid and CHO metabolism
- influences body mass
- influences mentation
Calcitonin
- functions
- fast action for blood calcium regulation
- lowers blood calcium levels
- antagonizes parathyroid hormone
Two isoforms of thyroid hormone
- T3 (triiodothyronine): most biologically active. High receptor affinity
- T4 (thyroxine): majority of thyroid secretion. Converted to T3 in target tissue by deiodination of T4
What is the stimulus for TH production?
TSH: binds to follicular cell receptors
What is first step in TH formation
- Production of thyroglobulin (protein without iodine)
- via protein synthesis
- Glycosylation in ER
- Packed into vesicles by Golgi
- Exocytosis from cellular apex
- goes into colloid
Second step to formation of TH
- Iodination of thyroglobulin
- follicle cells absorb iodides from circulation
- secreted to colloid
- thyroid peroxidase (on apical membrane) oxidizes iodide into iodine
- Iodine is added to thyroglobulin = monoiodotyrosin (MIT)
- Add another iodine to MIT = diiodotyrosine (DIT)
Coupling of thyroglobulins
- T3 = MIT + DIT
- T4 = DIT + DIT
What is the reservoir for TH?
colloid
- enough for 2 months without iodine in diet
Release of TH
- T3 and T4 have to be cleaved from the colloid by lysosomal enzymes
- endocytosis of colloid by follicle cell - released into circulation
Which TSH isotope is released in greater amount?
T4
How is TH metabolized?
liver, eliminated in bile (dt the iodine)
General overview of TH transport
- very little free
- most bound to protein
True/false: TH easily crosses the cell membrane
- FALSE
- polar molecule = not able to diffuse
- has to cross cell membrane via transporter
What is the TH receptor
THR: thyroid hormone receptor
What happens when TH binds THR
- in absence of TH, THR binds DNA and (usually) suppresses gene transcription
- TH binding = conformational change of THR which activates gene transcription
Action of TH
- increased BMR
- Calorigenesis (increased O2 consumption)
- Enhanced catecholamine activity by increasing adrenergic receptors
Energy production: - Increased glucose catabolism
- Lipolysis
How does TH affect cardiac output
Increases it by:
- increasing myosin
- increasing ion channels
- increasing beta adrenergic receptors
- increasing isotropy, chronotropy, and dromotropy
Table 16.2
has a lot of info on it i’m not putting on a NC :)
Calcium
- location of majority
- important fns
- 99% in bone
- Functions: blood clotting, neuromuscular activity, membrane permeability, cell secretory activity
