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Endocrine

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Question
Answer
More abundant; precursor to T3   T4  
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exerts majority of thyroidal hormone effects:   T3  
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Circulating T3 & T4   most is pro-bound; unbound = regulator for negative feedback inhibition mechanism  
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Thyroid hormones control:   O2 consumption CHO & protein metabolism, electrolyte mobilization & conversion of carotene to Vitamin A; also lipid synth & metab  
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Thyroid & insulin req in DM   Hypo: dec insulin req, inc chol/TG; hyper = inc insulin req, dec chol/TG  
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Amt TBG affects:   amt serum T3 & T4 (not physiologic thyroid status, which is affected by free hormone)  
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TBG can be measured by:   directly or by T3 uptake  
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Factors increasing TBG   Estrogens (inc serum TBG, so low T3 uptake value)  
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Factors decreasing TBG   Androgens or adrenocorticosteroids, low protein states (nephrotic syndrome or hepatic failure): dec serum TBG, so high T3 uptake value  
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T3 uptake =   indirect measure of pt’s Total T4; the higher the T3 uptake, the higher the pt’s T4 level  
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Thyroid state & T3 uptake:   Hyper: high T3 uptake; hypo: low T3 uptake  
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FTI (T7) =   total T4 x T3 uptake; FTI low: hypo; FTI high: hyperthyroid  
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Most common calculation method for free hormone   FTI (takes into account both absolute hormone level & binding capacity of TBG)  
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Hyperthyroidism labs   Depressed TSH; Elevated FT4, T3 Uptake, FTI  
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Hypothyroidism labs   Primary: High TSH; Low FT4, T3 Uptake, FTI. Secondary: low TSH & low FT4  
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Euthyroid, High TBG (eg, PG)   Sl low TSH; Free T4 nml or sl high; low T3 Uptake  
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Euthyroid, Low TBG (eg nephrotic syn)   TSH variable; Total T4 low to normal; Elevated T3 Uptake  
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ThRH stimulating test evaluates:   entire thyroid-pituitary-hypothalmus feedback loop  
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TSH levels don’t rise in:   primary hypopituitarism & states of altered thyroid homeostasis  
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TSI (thyroid stimulating immunoglobulin) test:   IgG Ab vs thyroid cell TSH receptors; 78% of Graves pts are positive. Test to monitor tx; titers rise with relapse  
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hypometabolism with low T3/T4; +/- low FTI & nml TSH; If reverse T3 (inactive product from T4 metabolic breakdown) is elevated =   euthyroid sick syndrome  
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Thyroid screening   USPSTF: no recs; ATA: TSH screen start at 35 yo & q5 yrs (sooner if risk factors or sx)  
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Thyroglobulin Ab >1:100 suggestive of:   autoimmune thyroiditis  
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Microsomal Ab >1:32 titer correlates with:   automimmune thyroiditis  
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Graves dz: predictors of remission   small goiter (decreases w/thianamides); FT3 predominance; neg TSI titer (IgG Ab vs thyroid cell TSH receptor)  
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TBG is increased / decreased by:   increased by estrogen, decreased by androgen  
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Effects of illness on T3 / T4   Severe illness or starvation decrease total T3 and free T3, increase rT3, no change of free T4 (euthyroid sick syndrome)  
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Thyroid imaging   Thyroid scan (99m Tc) (hot nodule = benign); US (good for nodules); CT; MRI; PET  
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Thyroid nodule: eval for neoplasm   TSH; FNA/bx essential; US (80%)  
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Hashimoto thyroiditis labs   Elevated thyroglobulin & peroxidase Ab; T4 increase > T3; low TSH. Elevated cholesterol.  
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Function of Ultrasound in thyroid disorder   to distinguish thyroiditis from Graves or nodule/goiter; to guide bx  
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medullary thyroid tumor labs   high calcitonin & CEA  
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TBG can be measured by:   directly or by T3 uptake  
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Factors increasing TBG   Estrogens (so low T3 uptake value)  
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Factors decreasing TBG   Androgens or adrenocorticosteroids, low protein states (nephrotic syndrome or hepatic failure): dec serum TBG, so high T3 uptake value  
🗑
T3 uptake =   indirect measure of pt’s Total T4; the higher the T3 uptake, the higher the pt’s T4 level  
🗑
Thyroid state & T3 uptake:   Hyper: high T3 uptake; hypo: low T3 uptake  
🗑
FTI (T7) =   total T4 x T3 uptake; FTI low: hypo; FTI high: hyperthyroid  
🗑
Most common calculation method for free thyroid hormone   FTI (takes into account both absolute hormone level & binding capacity of TBG)  
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Hyperthyroidism labs   Low TSH; Elevated FT4, T3 Uptake, FTI  
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ThRH stimulating test evaluates:   entire thyroid-pituitary-hypothalmus feedback loop  
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Thyroglobulin Ab >1:100 suggestive of:   autoimmune thyroiditis  
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Microsomal (peroxisomal) Ab >1:32 titer correlates with:   automimmune thyroiditis  
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TSH levels don’t rise in:   primary hypopituitarism & states of altered thyroid homeostasis  
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hypometabolism with low T3/T4; +/- low FTI & nml TSH   euthyroid sick syndrome  
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Graves dz eval   TSH, Free T4, Free T3; Thyroid uptake and scan (RAI)  
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Subclinical hypothyroid lab results:   increased TSH and normal T3 & T4  
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Effects of illness on T3 / T4   Severe illness or starvation decrease total T3 and free T3, increase rT3, no change of free T4 (euthyroid sick syndrome)  
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Thyroid functional assessment   131-Iodine Uptake; Measured at 24 hours; Normal 15-30%  
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Thyroid imaging   Thyroid scan (99m Tc); US (good for nodules); CT; MRI; PET  
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TSH levels do not rise in:   primary hypopituitarism & states of altered thyroid homeostasis  
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hypometabolism with low T3/T4; +/- low FTI & nml TSH   euthyroid sick syndrome  
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thyroid nodule: suspect neoplasm if:   Cold nodule on uptake scan. Solid lesion on u/s. High calcitonin  
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