It has been a belief for a long time that starving yourself would slow down the thyroid and not be productive in the long run if you want to lose weight, especially if you want to lose fat.
This was now confirmed in a recent study where the participants were randomly assigned to either a caloric restriction group, an exercise induced weight loss group, or a placebo group for 12 months (Weiss EP, et al, 2008).
The fat mass decreased significantly in both the caloric restricted group and the exercise group, but not in the placebo group. There were not a significant difference in fat loss between the caloric restriction group and the exercise group. What was interesting however was that the plasma concentration of the thyroid hormone T3 decreased in the caloric restricted group, but not in the exercise group even if both of the groups lost about the same amount of fat.
The lesson is that if you want to lose weight and stay healthy, exercise should be a part of the program in addition to a healthy diet.
Weiss EP, et al. Caloric restriction but not exercise-induced reductions in fat mass decrease plasma triodothyronine concentrations: a randomized controlled trial. Rejuvenation Res. 2008 Jun;11(3):605-9.