Thyroid Gland Flashcards
(27 cards)
What is the main anatomy of the thyroid gland?
- Thyroid cartilage
- Pyramid
- Thyroid gland —> 2 lobes
- Isthmus —> connect 2 lobes
- Parathyroid glands —> superior and anterior
What is the histology of the thyroid gland?
- Follicles —> colloid surrounded by follicular cells
- Parafollicular cells
Which nerve runs close to the thyroid gland?
Left recurrent laryngeal nerve —> vocal cords
What is the embryology of the thyroid gland?
- Thyroglossal duct develops from base of tongue
- Divides into 2 lobes
- Duct disappears leaving foramen caecum
- Thyroid gland being developing at 7 weeks
What are the 5 congenital anomalies of thyroid gland development?
- Thyroid agenesis —> no thyroid
- Lingular thyroid —> too high
- Retrosternal goitre —> too low
- Thyroglossal cyst —> lump
- Thyroglossal fistula —> hole
How does the thyroid gland produce T3 and T4? (6)
Blood —> Follicular cells:
- TSH bind to TSH-R on membrane —> stimulate TG
and TPO release
- Na+ and I- in via symport
Follicular cells:
- TG (thryoglobulin) released from nucleus
- TPO (thyroid peroxidase) released
Follicular cell —> Colloid:
- TG and TPO in
- I- in via antiport with Cl-
Colloid:
- I- to I via oxidation —> iodination of TG
- TPO + H2O2 —> catalyse iodination and coupling
- TG + I —> MIT (3-monoiodotyrosine)
TG + 2 I —> DIT (3,5-diiodotyrosine) - MIT + DIT —> T3
DIT + DIT —> T4
Colloid —> Follicular cells:
- T3/T4 in via endocytosis
- T3/T4 vesicles bind to lysosomes
Follicular cells —> Blood:
- T3/T4 released via exocytosis
Which 2 hormones does the thyroid gland produce?
- T3 (triiodothyronine)
- T4 (thyroxine)
What is T3?
Triiododthyronine (3 Is)
- MIT + DIT
- Bioactive when 2 Is on MIT
Reverse T3 (inactive) when 2 Is on DIT —> produced
by T4 deiodination in different place
- 20% direct thyroidal secretion
80% deiondinated T4
What is T4?
Thyroxine (4 Is)
- DIT + DIT
- Prohormone —> converted to T3
- Main product of thyroid gland
What happens to most of T4, how and why?
Converted to T3 (active)
- Deiodination via deiodinase enzyme
- T4 longer half-life (7-9 vs 2 days) because inactive
- Produced active or reverse T3
- 80% of T3
How are T3 and T4 transported round the body?
Bind to plasma proteins in blood
- TBG (thyroid-binding globulin) —> 70-80%
- Albumin —> 10-15%
- Transthyretin (prealbumin)
Some unbound —> bioactive
- 0.05% T4
- 0.5% T3
How to T3 and T4 affect cells?
T3 binds to TRs (thyroid hormone receptors) on TREs (thyroid response elements) in nucleus —> alter gene expression —> new proteins synthesised
What is the importance of thyroid hormones in a fetus?
Growth and development
- especially of CNS
What is cretinism and how is it screened?
Untreated congenital hypothyroidism
- Heel-prick test
What are the 5 functions of thyroid hormones?
- Growth
- CNS maturation
- Thermogenesis
- inc Na+/K+ ATPase —> inc O2 consumption —>
inc BMR —> inc heat release
- inc Na+/K+ ATPase —> inc O2 consumption —>
- Metabolism
- inc glucose absorption, glycogenolysis,
gluconeogenesis - inc lipolysis (catecholamines)
- inc protein synthesis and degradation
- inc glucose absorption, glycogenolysis,
- Cardiovascular function (catecholamines)
- inc cardiac output, heart rate, stroke volume
How are thyroid hormones regulated?
- TRH —> inc TSH —> inc T3 and T4
- Somatostatin —> dec TSH —> dec T3 and T4
- T3 and T4 —> dec TSH (negative feedback)
- I- —> dec T3 and T4 (Wolff-Chaikoff effect)
What is the Wolff-Chaikoff effect?
Regulation of T3 and T4 release from the thyroid glands via rejecting large quantities of I-
Which gender is more affected by thyroid diseases?
Women
- 4:1
What is primary hypothyroidism?
Underactive thyroid
- Autoimmune damage - Hashimoto’s thyroiditis
- T4 dec —> all released converted to T3 (no excess)
TSH inc —> not inhibited by T3/T4 release
What is Hashimoto’s thyroiditis?
Autoimmune disease —> hypothyroidism
- Antibodies bind to thyroid gland —> gradually
destroyed
What are the 7 main symptoms of hypothyroidism?
- Deeper voice - left recurrent laryngeal nerve
- Depression/tiredness - CNS
- Cold intolerance - thermogenesis
- Appetite dec but weight inc - metabolism
- Constipation - metabolism
- Bradycardia - cardiovascular
- Myxoedema coma - if untreated —> severe
What medical emergency can hypothyroidism lead to?
Myxoedema coma
How can hypothyroidism be treated pharmacologically? (3)
- Levothyroxine (synthesised T4)
- oral tablet —> 100 µg once daily (dose adjusted to
TSH levels) - side effects —> weight loss, headaches
—> heart attack, rapid heart rate
- oral tablet —> 100 µg once daily (dose adjusted to
- Liothyronine (synthesised T3)
- oral tablet —> multiple times a day (T3 shorter
half life)
- oral tablet —> multiple times a day (T3 shorter
- Combined thyroid hormone replacement (T3 + T4)
- oral tablet
- seen to improve wellbeing
- side effects (toxicity) —-> palpitations, tremour,
anxiety
—-> TSH dec
What is hyperthyroidism?
Overactive thyroid
- Autoimmune goitre - Grave’s disease —> all
- Toxic multinodular goitre
- Solitary toxic nodule —> 1
