Thyroid Function Tests in Hospitalized Patients

--low TSH (less than 0.3, but >0.05): usually due to nonthyroidal illness, generally euthyroid afer illness.  If actually <0.01, more likely to be due to true hyperthyroidism.  Can check freeT4, if low or normal, this is consistent with sick-euthyroid.  If high, may be overt hyperthyroidism (esp if TSH<0.01) OR may be artifically elevated due to decreased circulating thyroid-binding globulin.  If high/nl, can check a T3.  80% of T3 is made from peripheral conversion of T4--the enzyme that does this (5'monodeiodinase) is decreased in acute illness, and therefore inhibits fT4 conversion to T3. SO, a low T3 level supports a diagnosis of sick euthyroid.

--high TSH: can be transiently high during recovery from illness (up to 20 mU/L), usually nl on recheck.  If >20, likely true hypothyroidism.  Often fT4 and T3 are normal

--in the sickest ICU pts, majority have low TSH, low fT4 and low T3

--medications can cause abnormal TFTs without actual thyroid dysfunction

low TBG: androgens, glucocorticoids, niacin
high TBG: estrogen, tamoxifen, methadone, heroin, 5-FU
decreased T4 binding to TBG: salicylates, furosemine, some NSAIDs and heparin
increased T4 clearance: phenytoin, carbamazepine, rifampin, phenobarbital
suppressed TSH secretion: dobutamine, glucocorticoids
impaired conversion of T4-->T3: glucocorticoids, PTU, amiodarone

(Victoria Kelly MD, 6/25/10)