Can chlorine byproducts in your shower and tap water affect your thyroid?

What's actually in it

When water treatment plants add chlorine to kill germs, the chlorine reacts with natural organic matter in the water and creates disinfection byproducts (DBPs). The two main families are trihalomethanes (THMs) and haloacetic acids (HAAs). You absorb them by drinking tap water, but also through your skin and lungs during showers and baths.

Hot showers are a big exposure route. Heat turns THMs into a gas you breathe in, and warm water opens your pores for skin absorption.

What the research says

A 2026 study in Environ Sci Technol measured THMs and HAAs in the urine of study participants and compared those levels to thyroid function tests. The researchers looked at the mixed effect of all these chemicals together.

People with higher levels of disinfection byproducts in their urine showed altered thyroid hormone levels. The mix of THMs and HAAs appeared to interfere with how the thyroid makes and regulates T3 and T4 hormones.

The study used a framework that combined several statistical methods to separate the effects of individual chemicals from the cocktail effect. The mixture mattered more than any single byproduct alone.

You can't skip water disinfection, and you shouldn't. Chlorinated water prevents serious diseases. But you can reduce byproduct exposure with a whole-house or showerhead carbon filter. For drinking water, a countertop carbon or reverse osmosis filter removes most THMs and HAAs.