The Thy classification adopted by the Royal college of Physicians

• Thy1: Non-diagnostic (inadequate or where technical artefact precludes interpretation; smears must contain 6 or more groups of at least 10 thyroid follicular cells to be considered adequate).
  Action: FNAC should be repeated with or without ultrasound guidance.
• Thy2: Non-neoplastic (features consistent with a nodular goitre or thyroiditis).
  Action: Two diagnostic benign results 3-6 months apart required to exclude neoplasia. Decision to intervene surgically dependent on clinical factors (e.g. clinical suspicion and the presence of manifestations causing physical or psychological distress i.e. breathing difficulties or unfavourable cosmetic appearance).
• Thy3 (i): All follicular lesions.
  Action: Lobectomy. Complete thyroidectomy will be necessary if histology proves malignant.
  (ii): There may be a very small number of cases where the cytological findings warrant inclusion in this category rather than Thy2 or Thy4. This will be indicated in the report.
  Action: Discuss with cytopathologist to determine course of action.
• Thy4: Abnormal, suspicious of malignancy (suspicious, but not diagnostic, of papillary, medullary or anaplastic carcinoma or of lymphoma).
  Action: Surgical intervention indicated for differentiated tumour. Further treatment dependent upon pathology report. For lymphoma, metastatic tumour or undifferentiated i.e. anaplastic thyroid carcinoma, further investigation indicated.
• Thy5: Diagnostic of malignancy (unequivocal features of papillary, medullary or anaplastic carcinoma, or of lymphoma or of metastatic tumour).
  Action: Surgical intervention indicated for differentiated thyroid cancer, depending on tumour size, clinical stage and other risk factors such as gender and extremes of age. Appropriate further investigation indicated alongside radiotherapy/chemotherapy for anaplastic carcinoma, lymphoma or metastatic tumor.