Fast growing thyroid nodules - what do they mean?

Depends: Most thyroid cancer nodules do not grow fast. Most commonly a rapidly growing nodule is a cyst. But - you can't make this determination by yourself. Check with your primary doctor, an endocrinologist, or an endocrine surgeon. Thyroid ultrasound is usually the correct step.

"fast growing"?: Not sure what exactly you mean by "fast growing", but thyroid nodules can grow for several reasons. Often, they grow just because of the underlying reason for the nodules, which is a long list. Sometimes nodules grow because they bleed internally. Thyroid nodule growth may lead to nodule biopsy (or not). The decision cannot be made in a vacuum without a full exam.

Be evaluated: Most thyroid modules dont grow fast even those involving most thyroid cancers. Those that do should be evaluated as they can be concerning for aggressive thyroid cancer.