[Hereditary thyroxin-binding globulin deficiency--changed thyroid function tests]

Variation in concentrations of carrier proteins of hormones may influence the effect of the hormones and may cause confusion in the interpretation of laboratory results.

A Caucasian family with a hereditary thyroxin-binding globulin (TBG) deficiency was investigated. 22 persons in two generations had blood tests for TBG, thyrotropin (TSH), three-iodothyronin (T3), thyroglobulin (TG), thyroxin and for free thyroxin (FT4) by two different commercial tests, Delfia and IMx Abbott (IMx). Relevant health information was collected of all persons.

Six males had very low T4 values, non-detectable TBG, increased FT4 values on the Delfia test and within normal range on the IMx test. Six females had lower borderline T4 and TBG. All persons were clinical euthyroid.

The condition is considered to represent X-chromosome linked inheritance with hemizygote affected males and heterozygote female carriers with intermediate values for T4 and TBG. Commercial test kits for FT4 may present considerably different results in conditions with TBG deficiency. When a high level of measured FT4 combined with normal TSH is found; TBG deficiency should be considered.