Thyroid Gland Flashcards
What is the name of the bit that joins the 2 lobes of the thyroid gland
Isthmus
Where are parathyroid glands located
Embedded within the thyroid - parathyroid gland produces parathyroid hormone which is responsible for calcium metabolism
Why is it important to be extremely careful when operating on thyroid gland
Risk damage to the parathyroid which can cause problems with calcium metabolism and heart rhythm . If there is damage to the recurrent inferior laryngeal nerve ( which also lies close to the thyroid gland ) , there is a risk of damage to the vocals chords
Embryology of the thyroid gland
Midline outpouching from floor of the pharynx ( originates from the base of the tongue)
The thyroglossal duct then develops
Divides into 2 lobes
The duct disappears leaving the foremen caecum
Final position by week 7 of gestation
Thyroid gland then develops
What does the colloid contain
Contains thyroglobulin . Iodine atoms are incorporated into tyrosine residues on thyroglobulin and forms the bulk of colloid in follicles
Process of thyroid hormone secretion ( complicated diagram )
1) TSH arrived and binds to TSH receptors on thyroid glands
2) This also activates the sodium and iodide pump which pumps iodide ions ( e.g. from diet) into the follicular cell
3) Iodination occurs where iodide is oxidised to yield iodine
4) Prohormone thyroglobulin reacts with iodine ( iodination ) to form MIT and DIT
5 ) steps 3 and 4 are catalysed by thyroid peroxidase and Hydrogen peroxidase
7) MIT + DIT = T3 , 2x DIT= T4 ( coupling reaction)
8) packaged into lysosome and then T3 and T4 excreted into blood
Deiodination of triiodothyronine ( T3)
Thyroxine ( T4) is the main hormone produced by the thyroid gland
T4 is deiodinated to T3 , its bio active form in the target tissues
T4 may also be deiodinated in a different position to produce reverse T3 ( inactive)
What enzyme converts T4 to T3
Deiodinase enzyme
Circulating T3
80% comes from the deiodination of T4
20 % comes from direct Thyroidal secretion
T3 provides almost all the thyroid hormone activity in target cells
How is thyroid hormone transported
Mostly bound to plasma proteins
a) thyroid binding globulin ( TBG) - 70-80%
b) albumin ( 10-15%)
c) prealbumin ( aka transthyretin)
Only 0.05% T4 and 0.5% T3 is unbound ( bio active components)
Why is thyroid hormone especially important in babies
Essential for fetal growth and development in particular that of the CNS
Untreated congenital hypothyroidism ; called Cretinism when the baby is born without thyroid gland or inactive T3 . This required lifelong treatment
At birth TSH is measured in new born infants heel prick test on Day 5 of birth so very rarely seen now
Actions of thyroid hormone
Increases basal metabolic rate
Protein , carbohydrate and fat metabolism
Potentials actions of catecholamines ( e.g. tachycardia , lipolysis )
Effects on the GI , CNS and Reproductive systems ( look at diagram)
Half life of T4
7-9 days
Half life of T3
2 days
How is baby semi protected if has problems with thyroid in utero
Thyroid crosses the placenta from the mum so effects of not having enough thyroid is only seen after delivery
Control of thyroid hormone
Hypothalamus - TRH- anterior pituitary - TSH- Thyroid Gland - T3 and T4 which negative feedback on hypothalamus and anterior pituitary
Somatostatin released by hypothalamus can inhibit TSH
WolffChaikoff effecf
Iodine given in large quantities can inhibit T3 and T4
Clinically increased potassium iodide is given to someone with hyperthyroidism to stop the production of thyroid hormone
Why are thyroid disorder more common in women ( 4;1 ratio) ?
Most defects in the thyroid gland is to do with autoimmunity. Women are more predisposed to autoimmunity due to them needing to carry babies and so they are exposed to more antigen
Which is more common , overactive thyroid gland or under active thyroid gland ?
Both equal
Primary Hypothyroidism changes in thyroxine and TSH
Autoimmune damage to thyroid
- thyroxine levels decline
- TSH levels climb as a result of negative feedback
Commonest forms of hypothyroidism
Hashimoto’s thyroiditis ( hypothyroidism ) and Graves’ disease ( normally hyperthyroidism but can also be hypo )
Other diseases which can increase risk of hypothyroidism
The presence of one autoimmune disease can increase the risk of others ( e.g. vitiligo and pernicious anaemia)
Signs are symptoms seen in a patient presenting with hypothyroidism
Deepening voice Depression and tiredness Cold intolerance Weight gain and reduced appetite Constipation Bradycardia Eventual myoxedema coma Swollen face Dry rough skin Paresthesia Shaggy hair Hair loss Low sexual desire Cycle disorders BASICALLY SLOWS DOWN EVERYTHING
Pharmacology and therapeutics of hypothyroidism
Levothyroxine
Indications of levothyroxine
Hypothyroidism / radioactive iodine treatment
Hyperthyroidism ( blocking and replacement regimen )
Dosing of levothyroxine
Adjusted according to TSH ( aim to get into normal range) and T4
Common dose is 100 micro grams but may start lower for elderly who are at risk of heart attack
Potential complications of levothyroxine
Extremely rare and only happens if you take too much
Weight loss
Headache
Heart attach and rapid heart rate
Combined thyroid hormone replacement
T4 = prohormone , converted by deiodinase action to T3
Combination T4/ T3 - some have reported improvement in well being
Complicated by symptoms of toxicity - palpitations, tremor, anxiety - often combination treatment suppresses TSH
Hyperthyroidism
Thyroid makes too much thyroxine
Thyroxine levels rise
TSH levels drop
Causes of hyperthyroidism
Graves’ disease ( autoimmune disease ) where the whole gland is smoothly enlarged and the whole gland is overactive
Toxic multinodular goitre ( many nodules overproducing thyroid hormone )
Solitary toxic nodule ( single nodule overproducing thyroid hormone )
Graves’ disease
Autoimmune disease
Antibodies bind to and stimulate TSH receptor in the thyroid
- smooth goitre
Other antibodies bind to muscles behind the eye
- exophthalmos ( patients complain of dry or gritty eyes) . Can lead to severe eye complications and can need steroids
Other antibodies stimulate the growth of soft tissue of shins
- pretibial myxoedema
Signs and symptoms of hyperthyroidism
Heat intolerance Weight loss with increased appetite Myopathy Mood swings Diarrhoea Tremor of hands Palpitations Sore eyes, goitre Broken hair Hair loss Fragile fingernails
