Sarah is 55 years old and known to have Rheumatoid arthritis. She has noticed a central neck swelling. She feels the swelling has grown slowly over the last few months and denies any fatigue, change in bowel habit, hoarse voice or hair loss. On examination there is a firm thyroid goitre.
Subsequent tests reveal she is euthyroid but has both anti-thyroid peroxidase and anti-thyroglobulin (anti-Tg) antibodies.
What is the most likely diagnosis?