|
Disturbances of serum total T4 and T3 concentrations
represent a continuum of changes, primarily reflecting the severity of
nonthyroidal disorders, with the lowest values occurring in the sickest
patients regardless of the etiology of the underlying illnesses (Figure
1) (1,3).
Figure 1:
Free T4 values by direct equilibrium dialysis are normal or elevated in the majority of patients with nonthyroidal illnesses in the absence of hypothalamic-pituitary-thyroid disease or suppression by therapeutic administration of dopamine (4,5). Free T4 values determined by tracer equilibrium dialysis underestimate free T4 levels in patients with reduced serum T4 binding, including those with severe nonthyroidal illnesses (4,5). In contrast, changes in total reverse T3 levels are more disease specific being elevated in most patients with non-renal nonthyroidal illnesses, but normal or reduced in those with acute or chronic renal failure, nephrotic syndrome with normal glomerular filtration rates, primary hyperparathyroidism and autoimmune deficiency syndrome (AIDS) (1,2). Serum TSH levels tend to be reduced in severe nonthyroidal illnesses, and elevated during recovery from acute illnesses (1-3). Further, the inverse relationship between serum TSH and free T4 values appears to be altered in patients with nonthyroidal illnesses (Figure 2) (5). Figure 2:
However, many laboratory measurements of serum free T4 do not accurately reflect circulating levels (3-5), and serum TSH assays may measure immunoactive hormone that has varying bioactivity (7,8) , thus, obscuring the TSH-free T4 relationship in sick patients. |
