Once laboratory assay artefact has been excluded, and genuine hyperthyroxinemia with non-suppressed TSH is confirmed, judicious use of additional tests (including (free) T3 if not already measured (with calculation of the T4:T3 ratio), reverse T3 (rT3), thyroid autoantibodies, sex hormone-binding globulin (SHBG)) can provide important clues to the underlying diagnosis (Table 1) and guide the next phase of investigation (Fig. 1, Step 3) (7). Here, SHBG is linked to hyperthyroxinemia.