Thyroid, parathyroid and adrenal
Thyroid
- Endocrine gland: located: vertebral level C5 - T1: anterior neck
- Consists two lobes united by central isthmus
- Inferior thyroid cartilage, wraps around cricoid cartilage
Anatomy
Blood supply to thyroid gland
Nervous innervation
- Mainly receives innervation from autonomic nervous system
- Parasympathetic from branches Vagus nerve
- Sympathetic: cervical chain ganglion
- Effect of innervation on blood vessels - perfusion of thyroid
- Two main arterial supply: superior and inferior thyroid artery
- Superior thyroid artery derives: external carotid artery
- Inferior thyroid artery derives: thyrocervical trunk (branch of subclavian artery)
Venous drainage
- Drained by superior, middle and inferior thyroid veins
- Superior and middle thyroid veins drain ton internal jugular vein
- Inferior thyroid veins anastomose with each other and drain into left brachiocephalic vein
Embryology
- Thyroid develops base of tongue
- Embryological remnant foramen caecum
- Thyroid starts descent from base of tongue
- To base of neck at level of cricoid cartilage via thyroglossal duct:
- End 5th week, thyroglosssal duct degenerates
- 6th week., thyroid reached its destination an production of TSH by pituiatry detectable week 12
- Fetal thyroid hormone important for: neurodevelopment protection against maternal hypothyroidism
Histology
- Follicular cells
- Colloid
- Parafollicular cells
- cuboidal epithelium
Physiology
- Two main thyroid hormone
- T3: triiodothyroxine: more potent than T4
- T4: thyroxine: Longer half life than T3
- Follicular cells responsible for production and secretion
- Parafollicular cells: secrete calcitonin
- Stimulated by TRH and TSH
Thyroid hormone production
- Na+/I- symporters transport iodide ion for every sodium ion across plasma membrane
- Iodide converted to iodine by thyroid peroxidase in colloid of follicle
- 2 I- converted to one I2 molecule (more reactive)
- Iodine added to tyrosine residues on thyroglobulin molecules in colloid
- One iodine added: MIT: Two iodine added: DIT
- Coupling reaction then follows DIT + MIT= T3 and DIT + DIT= T4
- T3 and T4 still attached to thyroglobulin
Arrival of TSH
- TSH binds to TSH receptor follicular cell, endocytosis of thyroglobulin stimulated. From colloid into follicular cell
- Lysosomal enzymes cleave off thyroid hormones from thyroglobulin
- T3 and T4 diffuse out from cell and into bloodstream
Parathyroid gland
- Posterior lobes thyroid
- Parathyroid glands homeostasis of calcium
- Calcium levels drop or rise in blood, receptors in PT detect this and release or inhibit PTH
- Chief cells: PTH
- Oxyphil cells: no known function
PTH
- Calcium homeostasis
- Phosphate regulation
- Vitamin D activation
PTH + calcium homeostasis
- Hypocalcaemia: PTH cause bone resorption, liberating minerals (calcium into blood)
- Bone resorption done by osteoclasts. PTH cannot act on them, lack of PTH receptor
- PTH act on osteoblasts, indirectly stimulate osteoclast cells initiate bone resorption
Vitamin D
- PTH increases activity of 1 alpha hydroxylase: converts calcidiol (from liver) into calcitriol (active form vit D)
- Serum phosphate
- PCT kidneys: PTH reduces re-absorption of phosphates, decreasing conc of phosphate , Vit D can counterbalance effect
Adrenal cortex histology
mesoderm
Zona fasiculata
Zona reticularis
Zona glomerulosa
Adrenal medulla
neuroectoderm
Chromaffin cells: Secretion of catecholamine
- Noradrenaline (also a neurotransmitter)
- Adrenaline
Mineralcorticoids (aldosterone)
Glucocorticoids (cortisol)
- Stimulates gluconeogensis
- Enhances breakdown fat and protein
- Inhibits inflam response
- NA retention and K loss
STRESS
Androgens (sex hormones)
- Stimulates sodium reuptake: increase blood pressure
- Decrease K +, Increase H2O
Renin cleaves angiotensinogen
Angiotensin 1 - angiotensin 2 via ACE
- ACTH + ANG 2 stimulate aldosterone
- and..Low BP and low Na+
- High K+ aswell
Blood supply
Inferior phrenic , aortic and renal blood supply
Adrenaline
- Promotes breakdown glycogen - glucose
- Lactate to glcuose
- Lipolysis
- Increase cardiac muscle contraction
CRH - ACTH - Cortisol