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21 Cards in this Set
- Front
- Back
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Which dept orders the most thyroid tests?
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Psych
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T4 AKA thyroxine activation
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To active T3
Deiodination of the outer ring. |
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T4 AKA thyroxine deactivation
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Reverse T3 (inactive)
Deiodination of the inner ring. |
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Bulk of material secreted by thyroid
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T4 - which is more of a prohormone.
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Two huge places where deiodinases are present
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Anterior pit and liver.
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Are all thyroid responsive genes activated by T3?
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NO!
e.g. TSH alpha and beta |
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T3 and T4 mech of action
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T4 - enters cell and is conv to T3 in the cell.
T3 - freely enters the cell, binds nuclear receptors. Allows for a co-activator to bind and pushes off the co-repressor of the DNA. |
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Thyroid status determined by...
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free hormone (mainly T3) []
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How much T4 and T3 is protein bound?
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more than 99%
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Main circulating protein binding thyroxin or T3
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Thyroid binding globulin
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States of increased thyroid binding globulin
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These will raise TOTAL t4 levels - High total T4/T3
Pregnancy Estrogen therapy Acute hepatitis Congenital raised TBG levels Mutant albumin molecule |
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States of decreased thyroid binding globulin
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Low total T4/T3
androgens, corticosteroids, salicylates, barbs major illness/trauma nephrotic syndrome congenital decrease in TBG |
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Direct assay of free T4 and T3
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clinical assessment of the free thyroid hormone levels
Most commonly used, but in very ill pts it is not reliable. Uses an antibody that only recognizes the free/unbound hormone. |
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Equilibrium dialysis to assess free fraction
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clinical assessment of the free thyroid hormone levels
Gold standard and is expensive This spikes the pt with radioactive T4 and puts serum in dialysis bag where only unbound can get out. |
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Resin uptake test
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clinical assessment of the free thyroid hormone levels
Calculates the free hormone index, while direct assay or dialysis measures the actual []. This index is assumed to be proportional to the actual []. Spike sample with radioactive hormone, resin pellet competes with proteins for free hormone binding, and with more hormone in the pt, less resin is able to bind. |
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Raised free T4 and T3, but low TSH
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hyperthyroidism.
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Low free T4 but raised TSH
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primary hypothyroidism
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low free T4, low to normal TSH, pituitary changes
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secondary hypothyroidism
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Nuclear ways to test for thyroid function
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radioactive iodine uptake and radionuclide thyroid scan (I think this is good to catch cancers)
Most cold nodules on radionuclide imaging are benign, but all thyroid CAs are also cold. |
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Post-surgical pts or pts whoa re medically ill
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Production of T3 goes down due to downregulation of deiodination
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Measuring thyroid radioiodine uptake
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Good for hyperthyroidism
Pt takes a pill with radioactive iodine, put counter over neck and see what percentage was taken up by thyroid. Usually < 25%. High with hyperthyroidism (e.g. Graves) and low with pt taking exog thyroid pills, thyroiditis... |
