The thyroid
- Composed of follicles lined by cuboidal epithelium, which produce T3
and T4
- Within follicles T3 and T4 is stored bound to thyroglobulin
- When and as needed, they are secreted
- In circulation T3 and T4 are bound to:
- Albumin
- Thyroxine binding pre-albumin
- Thyroxine binding globulin
- Only about 1% of the hormones remain unbound
- These are physiologically active
- T3 is quick acting (hours)
- T4 is slow acting ( days)
- rT3 (reverse T3) is T3 produced in the peripheries from conversion
of T4
- Thyroid hormones promotes carbohydrate, protein and lipid metabolism
- Act on most cells of the body except the brain
- Increase basal metabolic rate and oxygen consumption
- Regulate tissue growth and development
Calcitonin
- Produced by parafollicular C cells
- Main action is to lower serum calcium
- Acts on skeletal tissue and bone
- Inhibits osteoclast activity and bone resorption
- Stimulates osteoblast activity
- Inhibits release of ionic calcium from bone
Parathyroid glands
- Number of parathyroid glands is variable but there are usually four
- Contain chief and oxyphil cells
- Chief cells produce parathyroid hormone
- PTH is important in controlling serum calcium
- PTH increases blood calcium levels
- Stimulates osteoclasts to release calcium from bone
- Increases renal resorption of calcium
- Promotes renal activation of vitamin D
- Increases absorption of calcium in the gut
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