Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1245 page)

   Recent rapid correction of hyperthyroidism or hypothyroidism

   Pregnancy

   Phenytoin therapy

   TSH may not be useful to evaluate thyroid status of hospitalized ill patients.

   Approximately 3 months of treatment of hypo- or hyperthyroidism; FT
₄
is test of choice.

   Lag time of 6–8 weeks is required for normalization of TSH after initiation of thyroid hormone replacement therapy.

   Dopamine or high doses of glucocorticoids may cause false normal values in primary hypothyroidism and may suppress TSH in nonthyroid illness.

   Rheumatoid factor, human antimouse antibodies, heterophile antibodies, and thyroid hormone autoantibodies may produce spurious results, especially in patients with autoimmune disorders (≤10%).

   Amiodarone may interfere with TSH.

   TSH is not affected by variation in thyroid-binding proteins.

   TSH has a diurnal rhythm, with peaks at 2:00–4:00
AM
and troughs at 5:00–6:00
PM
with ultradian variations. TSH levels vary diurnally by up to 50% and up to 40% variations on specimens performed serially during the same time of the day.

   Serum levels typically falls below 0.1 mIU/L during first trimester of pregnancy due to thyroid stimulatory effects of HCG and returns to normal in the second trimester.

THYROTROPIN-RELEASING HORMONE (TRH) STIMULATION TEST

   Definition