Eukaryotic genes generate multiple mRNA transcript isoforms though alternative transcription, splicing, and polyadenylation. However, the relationship between human transcript diversity and protein production is complex as each isoform can be translated differently. We fractionated a polysome profile and reconstructed transcript isoforms from each fraction, which we term Transcript Isoforms in Polysomes sequencing (TrIP-seq). Analysis of these data revealed regulatory features that control ribosome occupancy and translational output of each transcript isoform. We extracted a panel of 5' and 3' untranslated regions that control protein production from an unrelated gene in cells over a 100-fold range. Select 5' untranslated regions exert robust translational control between cell lines, while 3' untranslated regions can confer cell-type-specific expression. These results expose the large dynamic range of transcript-isoform-specific translational control, identify isoform-specific sequences that control protein output in human cells, and demonstrate that transcript isoform diversity must be considered when relating RNA and protein levels. Overall design: Total cytoplasmic and eight polysomal fractions of RNA were purified from HEK 293T cells in biological duplicate. Ribosomal RNA was depleted using Ribo-Zero (Human/Mouse/Rat; Epicenter) and libraries were prepared using the TruSeq RNA v2 kit (RS-122-2001; Illumina) skipping the polyA selection step. Reads are paired-end 75bp and sequencing adapters are GATCGGAAGAGCACACGTCTGAACTCCAGTCAC (read1) and AGATCGGAAGAGCGTCGTGTAGGGAAAGAGTGT (read2).