Endocrine System 3 Flashcards
Thyroid Gland
- lies anterior to the thyroid cartilage of larynx
- consists of 2 lobes connected by narrow isthmus
- **-only endocrine gland that stores secretoryproduct in large quantity (approx 100 days supply)
Clinical Note: Goiter
- an enlargement of the thyroid gland that is usually not associated with inflammation or cancer
- Simple goiter
- enlargement of one or both lobes
- deficiency of iodine in the diet
- iodine required to produce thyroid hormone
- Goiter belt
- areas deficient in iodine
- iodized salt
- Toxic Goiter
- grave’s disease
Thyroid Hormones
- T3 and T4 regulate the body’s metabolic rate (MR)
- have a calorigenic effect
- inc oxygen consumption
- inc heat production
- degrades
- liver glycogen stores
- fat stores
- muscle protein
- increase cardiac output and workload
- essential for normal growth and development
- act synergistically with growth hormone and may be necessary for synthesis of GH by pituitary
- stress inhibits thyroid axis
Control of thyroid hormone release
Negative feedback loop
- decreased levels of circulating thyroid hormone (or dec in body temp)
- TRH released from the hypothalamus
- stimulates anterior pituitary to release TSH
- stimulates thyroid gland to release biologically active T3 and T4 into bloodstream
- transported in blood bound to plasma proteins e.g.. thyroxin, albumin
Thyroid Gland
Functional units are follicles
-hollow spheres bounded by follicular cells
-spheres filled with colloid
-colloid’s main constituent is thyroglobulin synthesised by follicular cells
-follicles also synthesise and secrete thyroid hormones T3 and T4
T3 = triiodothyronine
T4 = thyroxine (tetraiodthyronine)
Synthesis and Storage of Thyroid Hormone
Synthesis and secretion of T3 and T4:
1. iodide trapping
-follicular cells actively transport iodide ions from blood
2. I- converted to activated iodide (I+) by enzyme thyroid peroxidase
3. one of two iodide molecules attached to thyroglobulin molecule in lumen
-coupling of T1 and T2
T4 = 2xT2 molecules
T3 = 1x T1 + 1X T2
4. under stimulus of TSH, follicle cells remove thyroglobulin by endocytosis
5. lysosomal enzymes break down thyroglobulin and release thyroid hormones into cytoplasm
6. T3 and T4 lipid soluble
-diffuse across basement membrane and enter bloodstream
-T4 usually secreted in greater quantity than T3 (T3 more potent and most t4 converted in body cells to T3 through cleavage of one iodine molecule)
7. transport in blood
-most combine with transport proteins, mainly Throxine-Binding Globulin (TBG)
Disorders
Cretinism -underproduction of thyroid hormone Myxoedema -decreased CO and HR, poor peripheral circulation Grave's Diseases -overproduction of thyroid hormone
Parathyroid Glands
4 small glands embedded in the posterior surface of the thyroid gland
Parathyroid Glands
2 cell types
- Principle (chief) cells
- secrete parathyroid hormone (PTH) - Oxyphil cells
- reserve capacity of PTH
- function largely unknown
Parathyroid Hormone
Function: one of three hormones important in Ca2+ homeostasis
Ca2+ homeostasis:
-increase Ca2+ = depresses neuromuscular activity
-decrease Ca2+ = over excitability (increase Na+ permeability, always opposite to calcium)
Effects of altered Ca2+ balance
-excitation contraction coupling in cardiac and smooth muscle
-tight junctions
-clotting of blood
(can be problematic, e.g.. change motility of digestive system)
Ca2+ homeostasis
levels maintained by controlling
- Ca2+ absorption from diet (will only absorb the amount that you need)
- Ca2+ loss in urine
- deposition of Ca2+ into bone (osteoblasts)
- resorption of Ca2+ from bone (osteoclasts)
3 Hormones controlling Ca2+ homeostasis
3 hormones control free Ca2+ levels:
- PTH
- Vitamin D3 (Calcitriol)
- Calcitonin
3 Hormones controlling Ca2+ homeostasis
PTH
PTH increases free Ca2+
- in bone (by resorption of Ca2+ and phosphate)
- in kidneys
- increase resorption of filtered Ca2+
- increase resorption of filtered phosphate
- activates calcitriol
3 Hormones controlling Ca2+ homeostasis
Vitamin D3
Vitamin D3 (calcitriol) increases free Ca2+
- in GI tract
- increase Ca2+ and phosphate absorption
- decrease Ca2+ loss in faeces
3 Hormones controlling Ca2+ homeostasis
Calcitonin works in opposite direction
- decreases levels of free Ca2+
- calcitonin released by C cells in thyroid follicles (peptide hormone)
- in bone: inhibits osteoclasts and increases Ca2+ uptake in bone
- in kidneys: by increasing secretion of Ca2+
- important in pregnancy and young children (where there is bone development and growth)
