When used sparingly as part of a varied diet, no harm is likely to result, but high protein meats should not be used as a staple. Turnips, soybeans, radishes, rapeseed, and mustard also contain goitrogens. These should be used rarely. However, the addition of kelp to the diet may reverse the goitrogenic effect of the brassicas. High purine foods, which include peas, beans, mushrooms, shellfish, beef, and organ meats, should be used sparingly because they can contribute to gout if overused.
Some tortoises apparently become fond of lettuce to the exclusion of other foods.
In my experience, all the turtles I've known preferred other foods over lettuce, but there are many people out there who feed lettuce almost exclusively. This is a recipe for a long miserable death. However, there may be some benefits in occasional feeding of lettuce. There is some evidence that it contains antiseptic properties. Additionally, celery or lettuce can be used to entertain a turtle that is already well fed.