It is stressed that in routine clinical diagnosis using I131the drink should not be greater than 10 μc and that rapidity in carrying out measurements is important. Physical methods are described for meeting these more stringent requirements. Backscatter measurements are described which show that the backscatter factor in measuring thyroid uptake increases for Geiger counter cathodes of higher atomic number. A double counter arrangement is described for reducing geometrical variations of thyroid position with only a single measurement. A more sensitive γ ray counting method for urine samples has been devised.