Triiodothyronine sera, values

Until recently it was not always possible to measure unbound thyroxine in serum directly because of the minute quantities involved. Therefore a free thyroxine index calculation was often made, since this value is proportional to the free thyroxine concentration. This index can be arrived at by multiplying a measure of thyroxine in the blood (usually the FBI) by a value derived from the measurement of thyroxine-unoccupied binding sites of thyroxine-binding proteins. This was possible since the uptake of thyroxine or triiodothyronine by red cells or resin is inversely proportional to the number of unoccupied binding sites. As thyroxine-binding sites become increasingly saturated, the free thyroxine levels increase. An example of this method of arriving at a free thyroxine index is the work of Clark and Horn (C5). Calculation of the free thyroxine index is valuable since the FBI determination and the resin uptake of Ts are determinations that are routinely made in many clinical laboratories. 

We and others have reported a series of studies in ammonium perchlorate production workers who are exposed intermittently to perchlorate, resulting in urine perchlorate values of up to 40 mg daily, and in normal volunteers given 3—3 5 mg perchlorate daily for 2 weeks or 0.5 and 3mg daily for 6 months (Lawrence et al, 2000, 2001 Greer et al., 2002 Gibbs et al., 1998 Braverman et al, 2005, 2006). Thyroid function studies, including serum TSH, thyroxine (T4), free T4, and total triiodothyronine (T3), were not affected by perchlorate exposure in the plant or perchlorate administration to normal volunteers, despite a decrease in the thyroid uptake of at the higher exposures. Despite a mean exposure of 3 years to high levels of perchlorate in the production workers, no abnormalities of the thyroid evaluated by ultrasound were detected compared to a nonexposed local population (Braverman etal., 2005). 

Figure 71.12 Thyroid hormone concentrations after iodine and seienium suppiementation. Mean ( SE) serum thyroxine (T4), triiodothyronine (T3) and thyrotropin (TSH) concentrations in the piacebo-iodine suppiemented subjects (O circies), and in the seienium-iodine-suppiemented subjects ( circies). The reference range for serum TSH concentrations is within the dotted iine. For serum TSH, bars indicate the vaiues 1 SE beiow and 1 SE above the mean on the iogarithmic scaie. A significant main effect of time was observed for serum T4 (P < 0.001), T3 (P < 0.001), and TSH (P = 0.011). Significantly different from all subsequent values, P < 0.001 (Heimert contrast). For serum T3, there was a significant main effect of group (P = 0.03). Reproduced with permission from Moreno-Reyes et al., (2003). The American Society for Nutrition.

Notes. The peroentage deorease in the serum thyroid hormone levels in Graves patient treated with oarbimazole is less in patients with higher urine iodine exoretion. Values are expressed in mean (standard deviation). T3 triiodothyronine, T4 thyroxine. 

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