Sir, Kayser, Perrild, Feldt-Rasmussen, Hegediis, Skovsted and Hansen (1991) have measured thyroxine (Tt) serum levels. Standing for 35 minutes increases venous hematocrit, haemoglobin and plasma protein levels by 10. 3%, 10.8% and 20.8% respectively (Hagan, Diaz and Horvath 1978). Standing for 90 minutes raises serum total calcium from 96.8 mg/1 to 100.2 mg/1 (Husdan, Rapoport and Locke 1973). Standing for 15 minutes increases plasma cholesterol concentrations by 10.4% and plasma triglyceride concentrations by 12.4% {Tan, Wilmshurst, Gleason and Soeldner 1973). Standing for 30 to 40 minutes increases vitamin B12 and folate plasma levels by more than 10% (Abalan, Toulet, Tignol, Lacoste, Brument, Vuillemin, Ducassou and Couzigou 1987). Standing for 45 minutes increases total phenytoin plasma concentration by 10.5% {Abalan, Vincon, Ellison, Sousselier, Demotes-Mainard, Lachiver and Albin 1990). The explanation is that when the subject stands, fluid (up to 23 % of plasma volume) (Fawcett and Wytin 1960) leaves the circulation under hydrostatic forces, particularly in the lower limbs (Hagan, Diaz and Horvath 1978;Waterfleld 1931) which swell (Waterfield 1931. This leads to an increase in concentrations of blood constituents such as red-cells, proteins and protein-bound substances (calcium, etc.), which do not readily pass through the capillary membrane. Reverting to the horizontal position reverses these changes. Changes are relatively stable 30 minutes after the modification of posture (Hagan, Diaz and Horvath 1978).T4 is highly bound to plasma proteins (more than 99.9 %) so it is highly likely that standing increases its total plasma concentration. To test this hypothesis, the following experiment was conducted.
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