The thyroid gland Flashcards
Cells in thyroid
Follicular cells - making up follicles
Parafollicular cells
Colloid inside follicles
Glands involved in calcium metabolism, nerve running close to thyroid
Parathryoid (4) glands, lie behind thyroid.
Left recurrent laryngeal nerve - supplies vocal chords
Embryology of thyroid gland
originates from base of tongue
development of thyroglossal duct and dividing into 2 lobes
duct disappears leaving foramen caecum
final position by week 7 then develops
Process of producing T3 and T4
Pituitary gland secretes TSH, binds to receptor in thyroid
Arrival of iodide ions through sodium iodide transporter, crosses cell to enter colloid. Oxidised to iodine.
TSH binding leads to production of prohormone thyroglobulin (tyrosine residues - iodinated)
TSH binding leads to activation of enzyme thyroperoxidase. With H2O2 catalyses iodination reaction with TG in colloid.
Leads to MIT and DIT, couple to T3 and T4.
TG moves back into follicular cell carrying T3 and T4, protein bonds broken down and T3 and T4 released into blood stream.
T4 to T3
T4 inactive, prohormone, converted by deiodinase present in target tissues enzyme into active T3. (Constitutes 80% of T3 in blood)
Transport of thyroid hormones in blood
Bound to plasma proteins
thyroid - binding globulin (80%)
albumin and prealbumin (15%)
only 0.5% unbound and active
Effects on gene expression
Binds to thyroid hormone receptor in nucleus.
Half life of thyroid hormones
T4 - 7-9 days
T3 - 2 days
Actions of thyroid hormone
Increase basal metabolic rate
Potentiate actions of catecholamines eg tachycardia and lipolysis
Effects on GI CNS Reproductive systems
Bone maturation
Control of thyroid hormone production
Negative feedback of T3 and T4 to hypothalamus and anterior pituitary.
Also somatostatin
Large quantities of iodide stop cellular production
Primary hypothyroidism symptoms
Deepening voice Depression and tiredness Memory problems Cold intolerance Weight gain and reduced appetite Constipation Bradycardia Myxoedema coma Swelling of face Goitre - enlargement of thyroid gland Low libido Period problems
Pharmacology and tretaments for hypothyroidism
Levothyroxine - identical to natural T4.
Can be used for hyperthyroidism with blocking and replacement.
Dosage adjusted according to TSH. Common oral dose 100 micrograms.
Potential side effects:
minor - weight loss, headaches
Major - heart attack, tachycardia
Causes of hyperthyroidism
Graves’ disease
Toxic multinodular goitre
Solitary toxic nodule
Graves’ disease
Autoimmune diease
Antibodies bind to and stimulate TSH receptor in thyroid.
Smooth goitre.
Other antibodies bind to muscles behind eye - bulging - exophthalamos.
Other antibodies stimulate growth of soft tissues of shins - pretibial myxoedema
Hyperthyroidism symptoms
Weight loss with increase appetite Goitre Heat intolerance Diarrhoea Mood swings Myopathy Tachycardia Tremor of hands Sore eyes
