Light micrograph section through human thyroid tissue showing thyroid nodules. Thyroid nodules are solid or fluid-filled lumps that form within the thyroid, a small gland located at the base of the neck, just above the breastbone. These lumps range from a single nodule to a cluster of them. Most thyroid nodules aren't serious and don't cause symptoms. However, a small percentage of these thyroid nodules can become cancerous.