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