Thyroid Gland Flashcards
What are the thyroid hormones?
T3 - tri-iodothyronine (active form)
T4 - thyroxine
What is the main function of the thyroid hormones?
Increased metabolic activity
On what basis are thyroid hormones secreted?
When TSH is secreted by the follicular cells
TSH is secreted when:
- NIS is upregulated so there is increased intracellular concentration of iodine (iodine trapping)
- Iodination of thyroglobulin
- thyroid peroxidase enzyme
What is the NIS?
Sodium iodine symporter
On the basolateral membrane of the follicular cells
What is Tg?
Thyroglobulin
Precursor protein of thyroid hormone
What is the function of the thyroid peroxidase enzyme?
Oxidises iodide ions to form iodine atoms
How are thyroid hormones synthesised?
1) NIS takes up iodide as TSH levels are high and Na+/K+ channels create a suitable gradient
2) Pendrin transporter allows iodide transportation into the colloid whilst follicular cells secrete thyroglobulin
3) Oxidation from iodide to iodine by thyroid peroxidase enzyme
4) Iodine binds to the tyrosine rings of thyroglobulin
5) Molecules will move back into the follicular cells whilst still bound to tyrosine
6) Lysosomes in the follicular cell bind with endosome containing thyroglobulin releasing tyrosine from the structure and so separating T3 and T4
7) MIT and DIT release iodine allowing the process to restart
8) T4 gets converted into T3 by thyroid deiodinase
9) T3 enters the nucleus of target cells and acts on thyroid hormone receptor initiating transcription for specific mRNAs increasing metabolic rate
What is the function of the colloid?
Where thyroid hormone synthesis occurs
What are MIT and DIT?
MIT - monoiodotyrosine (when 1 iodine binds to tyrosine ring of thyroglobulin)
DIT - di-iodotyrosine (when 2 iodines bind to tyrosine rings of thyroglobulin)
What is the function of thyroid deiodinases?
T4 to T3 conversion when it reaches target cells
How are thyroid hormones controlled?
- negative feedback so that when iodine levels are higher (iodide is lower) there is a lower rate of hormone production
- intracellular control through D enzymes
What is hyperthyroidism?
Hyper function of the thyroid gland
What are the causes of hyperthyroidism?
Autoimmune Grave's disease HCG Toxic adenoma Excess iodine Toxic multinodular goitre
What are the risk factors for hyperthyroidism
Infection
Stress
Female
What may hyperthyroidism cause?
Thyrotoxicosis
- when hormone released from a damage gland
What are the symptoms for hyperthyroidism?
CR: atrial fibrillation, dissipated excess heat
MET: increased protein and lipid degeneration, increased appetite, heat intolerance, hyperglycemia
NERVOUS SYSTEM: nervousness, seizures
SKIN: plummer’s nails, pretibial myxoedema
BONES: accelerated osteoclast activity, hypercalcemia, osteoporosis
HAEMATOLGOICAL: pernicious anaemia, B12 deficiency
EYES: eyelid retraction, inflammation of orbital soft tissue (bulging)
REPRODUCTION: oligomenorrhea, gynecomastia, ED
What is the diagnosis for hyperthyroidism?
High serum T3 and T4
What is the treatment for hyperthyroidism?
Thionamide drugs - carbimazole, propylthiouracil
Radioactive iodine
Thyroidectomy
What are the types of hypothyroidism?
Primary -> permanent loss/destruction of the thyroid
Central/secondary -> insufficient pituitary stimulation of a normal gland (most commonly thyrotroph cell damage from pituitary macroadenoma)
What are the causes of hypothyroidism?
Hasiomoto's disease Infiltrative disease Hypopituitarism Cabbage Lithium
What is the diagnosis for hypothyroidism?
Low serum T4, high TSH
What is the treatment for hypothyroidism?
Levothyroxine, liothyronine
What are the symptoms of hypothyroidism?
ENDOCRINE: delayed puberty, erectile dysfunction
MET: reduced BMR, decreased GLUT4 stimulation
RENAL: reduced GFR, mild hyponatremia
HAEMATOLOGICAL: normocytic anaemia, reduced cutaneous circulation so sensitive to cold, sinus bradycardia
GI: reduced appetite, weight gain
NERVES, MUSCLE, BONE: impaired fetal brain development, dementia, growth retardation
