ABSTRACT
This chapter discusses the current situation of the reference ranges for free T4 concentrations, [FT4], and TSH concentrations, [TSH]. Since the advent of thyroid diagnostics, attempts have been made to decipher the key relationships governing thyroid physiology to facilitate diagnosis, treatment, and monitoring of those with thyroid diseases. In July 1953, Sydney C. Werner wrote a research article in the Bulletin of the New York Academy of Medicine about the pituitary thyroid relationship in normal and disordered thyroid states. This was one of the first attempts to describe the influence of externally administered thyroid hormone on the thyroid gland behavior. The search for the relationship between FT4 and TSH got a boost with the introduction of automated high-throughput FT4–TSH assays and the application of statistical methods. The goal was to find serum concentrations of TSH and FT4 of the sampled healthy individuals deemed to have normal thyroid status representative of the human population.
