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Thyroid Labs

Endocrine

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