Does the thirst mechanism always underestimates fluid loss during work in the heat and after work ends?

YES.

The thirst mechanism always underestimates fluid loss during work, in the heat and after work ends. Therefore you should take frequent drinks through out the work period. If you drink 250mililitres every 15 minutes, you can replace one litre per hour. if sweat rate is higher you will find it extremely difficult to keep up with fluid needs. During prolonged periods of work in the heat, weight loss exceeds 2 percent therefore the person should rehydrate before returning to work.