Martha is a 40 year old receptionist who presents to the GP worried about a neck lump. She has noticed it over the last 2 weeks and denies any pain or growth. 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.
What is the most likely diagnosis?