Thyroid Flashcards
What is the thyroid important for?
Normal growth and development
Fetal development (maternal thyroid)
Metabolic activity and oxygen requirements especially brain
Regulates lipid and carbohydrate metabolism and thus body weight
Controlled by the hypothallamus and pituitary
Failure of the thyroid to develop properally can cause what?
Lingual thyroid (failure to migrate from the back of the tongue)
Thyroglossal cysts -Remnants of the thyroglossal duct - this move up with tongue protrusion. No need to worry.
Where is the thyroid gland, what innervates it and what supplies it with blood?
Deep to sternohyoid muscle
Trachea cartilage ring 2 and 3
Superior thyroid artery (branch of the external carotid) and the inferior thyroid artery (branch of subclavian)
Recurrent laryngeal nerve (posterior)
What is a good indicator of thyroid dysfunction?
Blood levels of thyroglobulin (precursor to thyroid hormone) it is dumped early in dysfunction, such as thyroiditis, goitre, differentiated thyroid cancer (cancer biomarker), factitious thyrotoxicosis (people taking thyroid hormone, causes low thyroglobin levels, can get it from eating burgers).
Dumping the store of preformed thyroid hormone can thyrotoxicity form the high levels, but then they get hypothyroidism because the stores take time to replenish.
What can be used to treat thyroid cancer?
Thyroidectomy followed by radioactive iodine
What are the cells in the thyroid and how can you look at them?
Take a fine needle biopsy of the thyroid.
Follicular cells (cuboidal, columnar active)
Parafollicular cells secrete calcitonin (elevated in medullary thyroid cancer)
What is a biomarker for medullary thyroid cancer?
Elevated calcitonin levels
Where does dietary iodine come from?
Soil to vegetables, milk vat cleaner, fortified foods, fish
What does thyroid deficiency cause?
Goitre
In pregnancy - cretinism due to low fetal thyroid levels - irreversible damage to the developing CNS
How does the thyroid trap iodine?
NIS (sodium iodine symporter) An active transporter that brings one iodine in against the concentration gradient and 2 Na molecules go in too.
There are low iodine concentrations in the blood but 50 days worth stored in the thyroid. Also a large store of preformed thyroid hormone.
NIS is how radioactive iodine enters the thyroid. However, some other tissues have NIS
Breast, gastric muscosa, ciliary body eye, salivary glands, differentiated thyroid cancer cells
How is thyroid hormone produced?
Iodide is taken from the blood into the follicle cell via NIS.
It is oxidized by thyroid peroxidase (can have autoantibodies against this enzyme and carbimazole inhibits this enzyme) to form iodine.
This allows the iodine to get into the colloid through the pendrin chloride transporter of the cell where it can bind to a tyrosine molecule in thyroglobulin. If one iodide is bound to tyrosine it is MIT (monoioddtyrosine)and if two binds it is DIT (diiodotyrosine). MIT and DIT or DIT and DIT bind together forming the thyroid hormones (throxine [T4] or triiosothyronine [T3]) still attached to thyroglobulin. This is then endocytosed into the cell in a colloid drop and the thyroglobulin is removed releasing T4 and T3.
Any unused MIT and DIT are recycled in the follicle cell to reuse the iodine.
What does carbimazoloe do?
Treat those with an overactive thyroid. It inhibits thyroid peroxidase and prevents iodine from getting into the thyroid cells to make thyrogloblulin
What can taking too much iodine do and how can doctors cause it?
Amiodarone (a medication for he heart) has a lot of iodine. Can cause the thyroid to stop working resulting in an underactive thyroid or it can cause the thyroid to become over active.
What is pendred syndrome and what are they symptoms?
Pendrin receptor takes iodine into the coloid butin the ear it takes chloride into the endolymph.
Mutation of this transporter results in deafness and hypothyroidism
Why do you get decreased thyroid stimulating hormone production in pregnancy?
Because the TSH, LH, FSH and HCG all have the same alpha subunit. HCG is elevated in pregnancy and this can actstimulate the thyroid into producing T3 and T4. The T3 levels are sensed by the pituitary and it responds by producing less TSH.
How is thyroid hormone production regulated?
The hypothalamus produces thyrotropin releasing hormone (TRH), which acts on the pituitary to produce TSH, which acts on the thyroid gland to produce T3 and T4.
T3 has a negative feedback on the pituitary and the hypothalamus.
What can be sen in the blood for different abnormalities?
Low TSH and high T4 (overactive thyroid causing high T4 and T3 and T3 is suppressing TSH).
Normal TSH and low T4 (caused by pituitary problem i.e. not getting stimulated to produce of TSH)
High TSH low T4 - thyroid not working - more TSH but no production of T4
High TSH high T4 - tumour producing TSH
What specifically does TSH do to increase thyroid hormone production?
Increases iodide into follicular lumen
Increases blood flow
Increases production of thyroglobulin production
Increases thyroid peroxidase
Increased endocytosis and degradation of the thyroglobulin
What is graves disease?
Autoantibodies against thyroid stimulating hormone receptor. Mimics TSH. Results in increased T4 and T3 production.
What does T4 do in the blood and tissues?
It is converted into T3. After athyroidectomy need to replaeT4 so that it can be converted to T3 for negative regulation of TSH.
What are the symptoms and signs of thyrotoxicosis?
Low TSH, high T4 and T3.
Symptoms Nervousness, anxiety, depression, psychosis Increased sweating Weight loss Heat sensitivity Tachycardia Weakness, paralysis due to hypokalaemia (in Maori and Asian men
Signs Bruit (noise caused by increase blood flow) over thyroid, tachycardia Goitre Skin changes Tremor Eye signs
What are the causes of an overactive thyroid (Thyrotoxicosis)?
Graves disease (most common)
Multinodular goitre: hyperfunctioning regions of thyroid and , not suppressed by circulating thyroid hormone.
Thyroditis
Drugs - aminodarone, iodine, thyroxine
Toxic nodule - self stimulating TSH receptor
What are the causes of an underactive thyroid?
Hashimoto’s disease - high TSH, low T4, antibodies,
autoimmune disease leading to thyroiditis (inflammation) - adult onset slow
Affects all organ symptoms
Decrease in energy metabolism - low basal metabolic rate
Decreased protein synthesis
What causes thyroid eye disease?
Autoimmune actvity in the back of the eye resulting in the production of glycosaminoglycans that goes into the fat of the eye pushing the eye forward.
Eye lids get retracted, then eye becomes red and ejected and inflamed.
