Background
At present, solitary toxic adenoma (STA) is preferentially treated with radioactive iodine (RAI) in a fixed or calculated dose. However, this may lead to undesirable hypothyroidism. Recently, radiofrequency ablation (RFA) has emerged as a promising alternative. Direct comparison between the two treatment options was however never performed. Therefore, we aim to assess the one year treatment effectivity and incidence of irreversible hypothyroidism of RAI and RFA. Secondary objectives are to assess, outcomes of different strategies of RAI, nodal volume reduction, treatment related events, quality of life and cost-effectiveness.

Study design
Multicentre, randomized controlled trial, nested within a prospective cohort study with a 5-year follow-up.

Study Population
Adult patients with hyperthyroidism caused by STA, either as a solitary node in an otherwise normal thyroid gland or as a single hyperactive node in a multinodular goiter, with an indication for treatment with RAI. Exclusion criteria include nodule size > 5 cm, and medical history of I-131 treatment.

Intervention
Patients agreeing to participate will be randomized to receive treatment with either RAI, or RFA (group 1 and 2). Patients declining randomization but agreeing to follow-up will receive the local standard treatment, which will usually be RAI (group 3).

Follow-up
Follow-up of patients will closely resemble current clinical practice. In addition, patients will be asked to fill out questionnaires on (thyroid related) quality of life, medical consumption and patient’s experience. Patients randomized to the RFA arm will undergo RFA according to current practice guidelines for non-functioning thyroid nodules, which is associated with a low risk profile. Patients randomized for RAI will be treated according to a calculated dose regimen, based on nodule uptake and volume with a maximum administered activity of 740 MBq.

Status
This study is open for both patient and participating centre inclusion, contact: RABITOstudie@rijnstate.nl. This study is registered with ClinicalTrials.gov, number NCT05142904.