Purpose The direct aqueous photolysis of the thyroid hormone levothyroxine (T(4)) has been studied. Methods and Result One of the major photoproducts, i.e., 4-[4-(2-amino-2-carboxy-ethyl)-2,6-diiodo-phenoxy]-penta-2,4-dienoic acid (P1), was isolated by liquid chromatography and structurally assigned by mass spectrometric (MS) and nuclear magnetic resonance spectroscopic methods. The identity of a second major product, i.e., 3,5-diiodo-l-thyrosine (P3), was confirmed through access to a commercially available standard. Furthermore, the structures of three additional transformation products are proposed on the basis of data obtained by high-resolution MS analyses. UV absorption spectra were determined for T(4) and the two photoproducts P1 and P3. Disappearance quantum yields were calculated for T(4) (I center dot = 0.014 at pH 12) and P3 (I center dot = 0.024 at pH 12 and I center dot = 0.010 at pH 8.5), whereas the compound P1 was found to be stable under the studied conditions (T(1/2) = 600 min). Conclusion The results indicate that solar UV light may have a significant impact on the fate of T(4) in the aquatic environment.