text: Prior to the availability of sensitive TSH assays, thyrotropin releasing hormone or TRH stimulation tests were relied upon for confirming and assessing the degree of suppression
  in suspected hyperthyroidism. Typically, this stimulation test involves determining basal
  TSH levels and levels 15 to 30 minutes after an intravenous bolus of TRH. Normally,
  TSH would rise into the concentration range measurable with less sensitive TSH assays.
  Third generation TSH assays do not have this
  limitation and thus TRH stimulation is generally not required when third generation TSH
  assays are used to assess degree of suppression.