No serological cut-off exists to separate low T3 syndrome (LT) and central hypothyroidism (CH). The objective of this retrospective study was to propose such a cut-off. The first participant group comprised 52 patients from the age of six to twenty years. This group consisted of patients of 36 anorexia nervosa with LT and 16 CH. The second participant groups comprised 229 patients of all the same range of ages at the same hospital and included LT (n = 58) and CH (n = 4) patients, respectively. The third group of participants comprised 125 LT and 27 CH patients at the same hospital at all ages less than eighteen years. The last group of participants comprised 10 CH patients from the other two hospitals. The main outcome measure was fT3/fT4 ratio (pg/mL, ng/dL respectively). This ratio in the first group was significantly different (p < 0.05) between LT and CH. When the cut off value of fT3/fT4 was set as 2.0, the sensitivity of the LT and CH patients in the second group was 62% and 100%, respectively. This cut-off value of 2.0 was useful for distinguishing LT from CH only above the age of two years, as shown in the third group. The fT3/fT4 in 10 subjects with CH in the last group, aged 2 to 7 years, ranged from 2.55 to 7.71. In conclusion, fT3/fT4 less than 2.0 suggests LT rather than CH for patients from the age of two to eighteen years.