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45 Cards in this Set
- Front
- Back
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How much of free/active T3 is made by the Thyroid?
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only 20%, the rest is converted from less potent T4
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Half lives of T3 vs T4?
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T4 = 7 days
T3 = 24 hours |
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TSH, T4, and T3 levels in Primary Hypothyroidism?
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TSH: High
T4: Low T3: Low |
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TSH, T4, and T3 levels in Primary Hyperthyroidism?
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TSH: low
T4: high T3: high |
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TSH, T4, and T3 levels in Secondary HypoThyroidism
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Low across the board
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TSH, T4, and T3 levels in secondary hyperthyroidism?
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High across the board
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TSH, T4, and T3 levels in tertiary hypothyroidism?
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Low across the board
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TSH, T4, and T3 levels in tertiary hypothyroidism?
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high across the board
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What is key to take into account when interpreting thyroid hormone results?
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PROTEIN BINDING!!
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So what does protein binding have to do w/ T4?
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If its bound it is INACTIVE
It must be unbound to be active Only 0.03% is unbound though |
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What does T4 bind to?
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TBG (75%)
Thyroxine binding Prealbumin (20%) Albumin (5%) |
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How much of T3 is unbound?
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0.3%
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How does T3 bind?
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Tightly to TBG (38-80%)
Very little to TBP (9-27%) Weakly to Albumin (11-35%) |
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Plasma concentrations of TBG, TBP, and albumin?
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TBG: 1-2 mg/dL
TBPA: 15-20 Albumin: 3000-4500 |
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What happens to T3/4 levels if Protein Concentration is changed?
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T3/4 levels change to maintain the normal ratio
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How do changing levels of T3/4 affect protein concentration?
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protein concentration doesn't change at all
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Drugs that can INCREASE TBG and thus T4/3?
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OCP's
methadone Heroin Tamoxifen |
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Conditions that can INC TBG and thus T4/3?
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Preggers
Hepatitis HIV Billiary Cirrhosis Acute Intermittent Porphyria Genetics |
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Drugs that dec TBG and thus T4/3?
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Glucocorticoids
Androgens Furosemide Salicylates |
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Conditions that dec TBG and thus T4/3?
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Genetics
Renal disease w/ significant protein loss |
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Which tests are usually the only ones you need?
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TSH*
Free T4 |
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What is TSH testing not good for?
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Monitoring Rx b/c there's an 8 week lag for axis to catch up
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So what is a better monitoring test?
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FT4
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Some not pathological reasons for low TSH?
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Elderly patients
Transiently w/ systemic illness |
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Why is FT4 hard to measure?
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b/c there's so little of it
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Abnormal TSH + Normal FT4 = ?
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Subclinical Thyroid Disease
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Normal TSH + Abnormal FT4 = ?
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usually protein binding abnormality
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So FT4 is tough to measure, so what is the second most ordered test after TSH?
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Total T4
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Why would you possibly want to measure T3 before T4?
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its a more sensitive early indicator of hyperthyroid conditions
i.e, it becomes elevated before T4 |
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Kicker for measuring T3 in hypothyroidism?
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It may be normal and T4 may be low b/c the body is trying to maintain T3 by converting it from T4
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What do acute illness and starvation do to T3?
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Decrease it
(sick thyroid syndrome) |
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what is T-Uptake and what does it measure?
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T3-Resin Uptake
It indirectly measures the Thyroid hormone binding capacity, NOT T3 |
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So what's the use of T-Uptake?
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When viewed w/ Total T4 it'll allow you to approximate Free T4
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Old T-uptake vs New?
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Old: RIA test
New EIA test |
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what the heck is Free Thyroxin Index used for?
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it'll tell you if the level of Total T4 is appropriate for the functional level of TBG...geez
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why not do it?
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b/c you can just do a daggum Free T4
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What does an elevated/decreased FTI mean?
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Elevated FTI = likely hyperthyroid
Dec FTI = likely hypothyroid |
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How to calculate FTI?
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Total T4 X % Uptake
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Thyroid Antibodies to look for?
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Thyroid Peroxidase Antibody
Thyroglobulin Antibody (not TBG) Thyroid Stimulating Ab |
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What's up with Thyroid Peroxidase Ab?
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Associated with Graves (hyper) 80% of the time
Ass w/ Hashimoto (hypo) less of the time but still ass |
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Kickers for Thyroglobulin Ab?
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80% ass w/ Graves
also Ass w/ hashimoto Can be found in normal people or after viral infections |
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Kickers for Thyroid Stimulating Ab?
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Strongly ass w/ graves
Predictor of relapse/remission Predictor of neonatal disease occasionally ass w/ hashimoto |
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usefulness for Thyroglobulin test?
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Managing Thyroid carcinoma
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4 causes of thyroid testing interference?
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Cross-Reactivity
Endogenous Analyte Ab's Heterophile Ab's Drug Interactions |
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Drug that can lead to increased Free T4?
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Heparin
it blocks binding of T4 to TBG |
