There are 12 words that start with 'thirs'.
| thirst | thirsts |
| thirsty | thirsted |
| thirster | thirsters |
| thirstier | thirstily |
| thirsting | thirstiest |
| thirstiness | thirstinesses |
Note: These 12 words that start with 'thirs' are found from the USA & Canada version of the tournament Scrabble word list.