Abstract Background Horner's syndrome (HS) presenting with a neck mass is a rare but challenging clinical scenario which may be caused by malignant thyroid disease. Methods A concise review of the literature (PubMED database; 1990–2013) on the clinical management of neck masses with HS. An example case is also discussed. Results 1.3% of HS is caused by thyroid pathology. Thyroid pathology is the commonest cause of a neck mass associated with HS: the majority are caused by benign pathology; with carcinoma and lymphoma accounting for the remainder. Anaplastic thyroid carcinoma (ATC), thyroid lymphoma (TL) and thyroid sarcoma (TS) typically present with rapidly enlarging anterior neck masses in the elderly and are difficult to distinguish clinically. Although fine needle aspirate cytology (FNAC) is the diagnostic tool of choice for thyroid masses, core or incisional biopsy may be necessary when FNAC is inconclusive. Conclusion Differentiation between ATC, TL and TS is imperative as their treatment and prognoses differ greatly. Where feasible a combination of surgical debulking, radiotherapy and chemotherapy is the treatment of choice in ATC. Advanced cases benefit from 2 monthly endoscopic surveillance ± tracheostomy, stenting or Nd-YAG laser therapy. Aggressive oncological resection alone is recommended in TS. Treatment regimes in thyroid lymphoma (typically chemotherapy ± radiotherapy) differ for histological sub-types. 5-year failure-free survival is 90% in TL compared with a mean survival of 6–8 months in ATC and 10months in TS.