go to the bathroom
Synonym discussion
Sense: idiom/1
All four phrases mean the same basic action — using the toilet — but differ in register and regional use. 'Go to the bathroom' is the standard polite expression in American English, suitable in nearly any situation. 'Use the toilet' is more direct and medically neutral, common in instructions and formal writing. 'Go to the restroom' is typically heard in public spaces (restaurants, airports, theatres) and carries a slightly more formal tone. 'Go to the loo' is exclusively British and informal — fine among friends but not in a job interview. For example, a teacher might say 'Who needs to go to the bathroom?' to young children, while a doctor would ask 'Do you need to use the toilet?' and a British colleague might say 'I'm just going to the loo.'
Etymology
The phrase 'go to the bathroom' came into common use in the early 20th century, when indoor plumbing made bathrooms a standard feature of homes. The word 'bathroom' originally meant a room for bathing, but because the toilet was typically in the same room, the phrase became a polite way to refer to using the toilet without mentioning it directly.