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Thyroid Labs
Endocrine
| Question | Answer |
|---|---|
| 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
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