Martha is a 40 year old lady presents to the GP with a neck lump, which she noticed 2 weeks ago. It has not changed during that time. She is a non-smoker and drinks no alcohol. There is no hoarse voice or dysphagia.
On examination she has a 1cm non-tender mobile lump in the midline of the neck. It moves upwards when you ask her to swallow a sip of water but there is no movement with tongue protrusion.
What is the most likely diagnosis?