Prevalent low serum thyroxine-binding globulin level in Western Australian aborigines: its effect on thyroid function tests.

The first major variation from the normal human levels of thyroxine-binding globulin (TBG) in a racial group is reported in full-blood Aborigines of Western Australia. This finding has not only biochemical and anthropological interest, but also medical significance in that many common laboratory tests of thyroid function in this group are very liable to misinterpretation because of low TBG levels. In Aborigines in the Halls Creek area of Western Australia, 40% have TBG levels below the lower reference limit for Caucasians, and 18% have levels between half and one-third of the lower limit. Such subjects have low thyroxine levels by Caucasian standards, and frequently the results of triiodothyronine uptake (T3U) tests lend support to the false suggestion of hypothyroidism. Caucasian reference ranges are not valid in this large proportion of the Aboriginal population. It is not yet clear whether the low TBG levels are a normal feature in this racial population, or whether there is some other cause. The subjects of the study were not acutely ill and Aborigines with low TBG are well distributed throughout the State.