Some of the sodium in salad dressing comes from salt. Additionally, some brands add sodium-containing flavor additives, such as MSG and its cousins, disodium inosinate and disodium guanylate. In a review of major brand-name foods sold in US stores, salad dressing averaged 304 mg of sodium per 2-tablespoon (28-gram) serving, or 13% of the RDI.
However, sodium ranged from 10–620 mg per serving across the samples of salad dressing, so if you shop carefully, you could find one low in sodium. An even better option is to make your own — try using extra virgin olive oil and vinegar.