Reference ranges of serum TSH, FT4 and thyroid autoantibodies in the Thai population: the national health examination survey.

OBJECTIVE: Data on reference intervals of thyroid functions in Southeast Asia are limited. The aim of this study was to provide reference ranges of thyroid functions and thyroid autoantibodies in Thais.

DESIGN AND METHODS: Serum samples from 2545 apparently healthy non-pregnant subjects, aged ≥14 years, from the fourth Thai National Health Examination Survey were measured for TSH, FT4, antithyroperoxidase (TPO Ab), antithyroglobulin (Tg Ab) and antithyrotrophin receptor antibodies (TRAb). A reference population was selected from the disease-free population by excluding those who had thyroid autoantibodies and TSH > 20 mIU/l.

RESULTS: For the total population, median TSH and FT4 levels were 1·94 mIU/l and 1·35 ng/dl, respectively. TSH was higher and FT4 was lower in females than in males. Based on National Academy of Clinical Biochemistry criteria, the reference intervals (2·5th-97·5th percentile) were: TSH, 0·34-5·11 mIU/l; FT4, 0·98-1·79 ng/dl; TPO Ab, 5-84·88 IU/ml; Tg Ab, 10-118·2 IU/ml; and TRAb, 0·3-1·24 IU/l. With the new reference ranges, hypothyroidism was found in 4·16% of the total population (0·78% overt and 3·38% subclinical hypothyroidism) and hyperthyroidism was found in 3·18% (0·94% overt and 2·24% subclinical hyperthyroidism). Positive TPO Ab, Tg Ab and TRAb were found in 8·96%, 12·26% and 5·93%, respectively. The upper normal limit of TSH tended to increase with age, particularly for those aged 80 years and older.

CONCLUSIONS: The reference interval for TSH needs to be derived from each specific population. Slightly elevated TSH concentrations in the elderly could be considered acceptable, with no need for thyroxine treatment.