Interrelations Between FT3, FT4 and Pituitary TSH

TSH plays a central role in current thyroid function testing both as a diagnostic tool and therapeutic target. Recent studies have suggested a more complex and hierarchical relationship between logTSH and FT4 over the entire functional spectrum than the widely assumed single log linear gradient (1-4). Our group has also shown a disjoint between pituitary TSH and FT3 serum levels being operative under conditions of levothyroxine monotherapy (5). The present prospective observational study aims at confirming some of these findings and exploring additional factors other than TSH that may be important in shaping the interrelation of thyroid parameters and modulating thyroidal activity in health and disease. To this purpose, unselected patients presenting for thyroid testing or treatment to a specialised unit at a teaching hospital will be prospectively studied to assess the interplay of FT3, FT4 and TSH under various conditions, and to evaluate various thyroidal and non-thyroidal influences, such as disease entity, thyroid volume, deiodinase activity, thyroid medication, surgery, radioiodine treatment, age, BMI, smoking on pituitary set-point and homeostatic equilibria.