Thisin vitro method provides information on the TPO-mediated iodination of tyrosine. The conversion from L-tyrosine to Iodotyrosine by TPO that is present in prepared extracts from FTC-238 human follicular thyroid carcinoma cells transfected with human recombinant TPO, is measured with LC-MS/MS. Here, TPO is linked to thyroid gland follicular carcinoma.