Measurements of serum thyrotropin (TSH) and free thyroxine (T4) concentrations were conducted in infants, children, and adults to assess maturation of the hypothalamic-pituitary-thyroid (HPT) feedback control axis. Serum free T4 and TSH concentration data were collated for cord blood of the midgestation fetus, for premature and term infants, and for peripheral blood from newborn infants, children, and adults. Mean values were plotted on a nomogram developed to characterize the reference ranges of the normal axis quantitatively based on data from 522 healthy subjects, 2 weeks to 54 years of age; 83 untreated hypothyroid patients; and 116 untreated hyperthyroid patients. Samples for 75 patients with thyroid hormone resistance were also plotted. The characterized pattern of HPT maturation included a progressive decrease in the TSH/free T4 ratio with age, from 15 in the midterm fetus, to 4.7 in term infants, and 0.97 in adults. Maturation plotted on the nomogram was complex, suggesting increasing hypothalamic-pituitary T4 resistance during fetal development, probably secondary to increasing thyrotropin-releasing hormone (TRH) secretion, the marked, cold-stimulated TRH-TSH surge at birth with reequilibration by 2-20 weeks, and a final maturation phase characterized by a decreasing serum TSH with minimal change in free T4 concentration during childhood and adolescence. The postnatal maturative phase during childhood and adolescence correlates with the progressive decrease in thyroxine secretion rate (on a microg/kg per day basis) and metabolic rate and probably reflects decreasing TRH secretion.

Using valproic acid as an example, the authors demonstrate that drug response signatures derived from genome-wide expression data can identify individuals likely to respond to a drug, and propose that this method could select optimal populations for clinical trials of new therapies.

Systemic inaccuracies, proportional to the concentrations of serum proteins and the thyroxine (T4) they carry, have been reported in direct free T4 immunoassays. However, analytical recoveries of free T4 have not been carefully examined in most current methods, and they have not previously been examined across the pathophysiological range of serum T4 binding. In the present study we investigated ranges of serum T4 binding using free and total T4 measurements from 1359 individuals. Carefully characterized, gravimetrically calibrated, serum-based free T4 test solutions were then prepared with a constant normal free T4 concentration (12 ng/L) and varied serum T4 binding (approximately 300:1 to 24,000:1, ng protein bound T4: ng free T4). These standardized test solutions were analyzed using five T4 analog based free T4 methods. Analytical recoveries were calculated as ratios of actual free T4 measurements to the target value, and expressed as a percent of the target. Analytical recoveries were directly proportional to the extent of serum T4 binding and ranged 2% to 155%, 25% to 131%, 53% to 106%, 37% to 93%, and 37% to 73%, lowest to highest, in different methods. These systemic inaccuracies will confound interpretations of free T4 test results in clinical conditions with altered T4 binding. Future investigations into free T4 status must examine the analytical recovery of the free T4 method(s) used, as they relate to the extent of serum T4 binding in the clinical condition(s) studied.

BACKGROUND:Analog-based free testosterone test results, sex hormone binding globulin (SHBG) concentrations, and total testosterone concentrations are somehow related. This study used new experiments to clarify these relationships. METHODS:An analog-based free testosterone immunoassay and a total testosterone immunoassay were applied to well-defined fractions of serum testosterone. First, they were applied to the 2 fractions (retentate and dialysate) of normal male serum obtained by equilibrium dialysis. Second, they were applied to covaried concentrations of SHBG and total testosterone. Third, they were applied to decreasing concentrations of SHBG and protein-bound testosterone, offset by increasing concentrations of protein-free testosterone, while total testosterone was held constant. RESULTS:The analog-based free testosterone assay and the total testosterone assay detected and reported serum testosterone test results from serum retentate, whereas neither assay detected the free testosterone in serum dialysate. Test results reported by the analog-based free testosterone assay followed varied concentrations of SHBG and total testosterone. When total testosterone was held constant, however, analog-based free testosterone test results did not follow varied concentrations of serum proteins or of free testosterone.CONCLUSION: An analog-based free testosterone immunoassay reported free testosterone test results that were related to total testosterone concentrations under varied experimental conditions. This alleged free testosterone assay did not detect serum free testosterone (the test results it reported were nonspecific) and should not be used for this purpose.

Excessive bias and imprecision are major analytical problems associated with some assays for free and total thyroxine (T4). Bias in free T4 methods is largely proportional to variations in serum T4 binding. In direct methods, this is attributable to requirements for substantial quantities of protein-bound T4 to replace analytical losses of free T4. In some total T4 methods, bias is inversely proportional to the amount of serum T4 binding and is attributable to the incomplete release of serum protein-bound T4. In others, bias is fixed and attributable to inaccurate calibration. Manufacturers should report the bias in their methods. Calibrations should be standardized. Imprecision varies widely among methods, but is generally less for total T4 methods than for free T4 methods. A consensus on quantitative analytical performance goals for free and total T4 methods would be helpful. Here, performance goals are proposed, based in part on the best achievements of current methods.