A thyroid cyst is a fluid-filled sac sitting inside the thyroid gland. Most are harmless water balloons, so symptoms are usually mild and show up only when the cyst gets big. Neck lump is the star. You notice a smooth, round bump at the base of your throat that bobs up when you swallow. Size can change. The lump may swell after a cold or shrink on its own, making it seem to “come and go.” Pressure or fullness is common. You feel like something is stuck in your throat, especially when looking up or wearing a tight collar. Hoarseness... Learn more