Thyroid - Normal and Diseased Flashcards
Describe the thyroid-pituitary/hypothalamic axis. (including negative feeback loop)
The T4 and T3 in peripheral tissues would suppress secretion of both thyroid-releasing hormone and thyroid-stimulating hormone
Describe the embryology of the thyroid gland.
Migrates down from base of tongue to in front of trachea splits via thyroglossal duct, forming left and right lobe joined by isthmus
What hormones are produced in thyroid?
T3, T4, calcitonin; parathyroid hormone by parathyroid
What is the arterial supply of thyroid?
Describe the basic functional unit of thyroid
What are the molecular features of thyroxine (T4) and tri-iodothyronine (T3)?
Describe biosynthesis of thyroid hormones
6 steps!
Colloid and follicular cells
Colloid – synthesis of thyroglobulin; addition of iodine onto thyroglobulin to form MIT and DIT
Follicular cells – uptake of iodide, oxidising into iodine, iodine then transported into colloid
MIT + DIT –> T3
DIT + DIT –> T4 (thyroxine)
T3 & T4
Thyroxine make up 90% of thyroid hormones
Triiodothyronine makes up 10%
T3 is 4 times more potent than T4
T4 can be converted to T3 by liver & kidney
T3 is the active form, T4 is more of a storage
How are thyroid hormones regulated by TSH?
What is the role of thyroglobulin?
as a precursor to form thyroid hormones
What is the role of thyroxine and what are the biological effects of thyroid hormones?
On neurones and all cells - increased metabolic rate
*Carb/lipid/protein metabolism
On liver
On lungs
On heart
How are thyroid hormones degraded?
De-iodinases
Predominantly degraded by D1
How are thyroid hormones transported in plasma?
They are almost entirely bound by plasma proteins (thyroid binding hormones)
What are some thyroid hormone dysfunctions?
Hyperthyroidism
- Everything is going fast, anxiety
Hypothyroidism
- tend to get weight gain, constipation, lower mood, fatigue
Primary and secondary failure of thyroid causing hypothyroidism
Primary gland failure - may be associated with enlarged gland (goitre)
Secondary to TRH or TSH (no goitre)
Myoxesim and
Common cause of hyperthyroidism and symptoms?
Increased BMR
What are observations in primary hypothyroidism and hyperthyroidism?
1st hypo: low T3, low T4, high TSH
1st hyper: high T3, high T4, low TSH
What are observations of secondary hypothyroidism/hyperthyroidism? (hypothalamic/pituitary disease)
2nd hypo: low T3/T4, TSH low (or normal)
2nd hyper: high T3/T4, TSH high (or normal) –> loss of negative feedback
What is myxoedema and pretibial myxoedema?
Myxoedema –> severe hypothyroidism
Pretibial myxoedema –> rare clinical sign of Grave’s disease (an autoimmune thyroid disease that result in HYPERthyroidism!)
What are causes of primary hypothyroidism?
Goitrous
Non-goitrous
Self-limiting
What is autoimmune hypothyroidism and what are characteristics of it?
Autoimmune destruction of thyroid gland
Antibodies against thyroid peroxidase (TPO)
T-cell infiltrate and inflammation microscopically
What are clinical features of hypothyroidism? (very systemic)
What are autoantibodies relevant in thyroid disease?
How is hypothyroidism managed?
levothyroxine (T4)
What is myxoedema coma? (extreme of HYPOthyroidism)
Mortality up to 60%
Complete slow down of metabolic rates
Co-existing adrenal failure in 10% patients (as hypothyroidism can be very frequently autoimmune, it is very likely that Addison’s go along with it)
What is thyrotoxicosis and hyperthyroidism?
Thyrotoxicosis is the clinical, physiological and biochemical state arising when the tissues are exposed to excess thyroid hormone
Hyperthyroidism refers specifically to conditions in which overactivity of the thyroid gland leads to thyrotoxicosis
What are symptoms of thyrotoxicosis?
Vision -
*lid retraction
*double vision
*proptosis (Grave’s)
What conditions are associated with hyperthyroidism?
What is Grave’s disease and what are investigation findings of the condition?
positive TSH autoantibodies (TRAb)
Young patients, particularly females
What are clinical signs specific to Grave’s?
Thyroid eye disease (you cannot see white above cornea usually, but their eye balls are very protruding due to oedematous eye muscles)
What is nodular thyroid disease?
negative TRAb antibodies
Older patients
More insidious onset
Thyroid may feel nodular
What is thyroid storm? (extreme of HYPERthyroidism)
how is hyperthyroidism treated?
antithyroid drugs - inhibition of thyroid peroxidase (TPO) thereby blocking thyroid hormone synthesis
*Carbimazole or propylthiouracil
beta-blockers
*Propranolol
Radioiodine
What are subclinical thyroid diseases?
Abnormal
What is sick euthyroid syndrome?
