An enlarged thyroid, known as goiter, is often a sign of thyroid disease. Because an enlarged thyroid can indicate a number of different conditions, including thyroid cancer, one should consult a physician for an examination. While goiters are typically a thyroid disease symptom indicating either hyper- or hypothyroidism, they can also be the result of a nutritional deficiency, a lack of iodine in the diet. Iodine deficiency affects nearly one billion people worldwide but is rare in more highly developed countries where steps have been taken to include iodine in a regular diet.
Again, have your parents contact a doctor.
we have a Thyroid Disorders forum on Med Help I think you will find an answer from the good people there .good luck.