Is it safe to refreeze food once it has been thawed?

Yes. As long as the product stays below 40 degrees Fahrenheit (F) it is in the safe zone. Thawing in the refrigerator is best. If you use running water to thaw make sure the water stays below 70F and it has somewhere to go (ie. down the drain). You don't want to thaw in a pool of water that is not being replaced constantly. The Health Department (Kansas) gives product a 4 hour shelf life once it has left the safe zone (ie. above 40F). If you thaw product at room temperature be aware of this rule. Use a bio thermometer to check the temp of your food if you are not sure. The product may still be cooked and eaten within this time frame. I do not recomend using the microwave to thaw unless you plan on eating it right away. The microwave uses heat to thaw and is inconsistant in how it affects the product and parts of it may get above 40F.

Watch out for shellfish. Raw oysters on the half shell are the most dangerous thing the Health Department will allow in restaurants in the US, they need to stay as close to frozen as is edible. When shellfish die they become poisonous, this is why lobsters are cooked while they live. Frozen raw shrimp can be thawed and refrozen unless they leave the safe zone (ie. above 40 F).

Thawing then refreezing is a quality issue. Water and oils are lost which amounts to losing flavor (though freezing them in the first place has already done this) and degrades texture.