Jerald C. Nelson

Reference ranges for newer thyroid function tests in premature infants

OBJECTIVE To establish reference ranges for recently developed assays of thyroid function in premature infants. METHODS We measured serum free thyroxine (T4) by direct equilibrium dialysis and serum thyrotropin by a sensitive immunometric method in 104 preterm infants (25 to 36 weeks of gestational age) during the first week of life. RESULTS The free T4 level correlated positively with gestational age (p < 0.0001; r2 = 0.09) and differed significantly between adjacent gestational age groups (p < 0.05). Free T4 concentrations (mean +/- SD) for the 25- to 27-, 28-to 30-, 31- to 33-, and 34- to 36-week groups were 18.0 +/- 5.2, 25.7 +/- 9.0, 30.9 +/- 9.0, 36.0 +/- 10.3 pmol/L (1.4 +/- 0.4, 2.0 +/- 0.7, 2.8 +/- 0.8 ng/dl), respectively. Two reference ranges for free T4 were determined, one for 25 to 30 weeks (6.4 to 42.5 pmol/L (0.5 to 3.3 ng/dl) and one for 31 to 36 weeks (16.7 to 60.5 pmol/L (1.3 to 4.7 ng/dl)). The logarithm of the value for thyrotropin correlated positively with gestational age (p < 0.001; r2 = 0.08); one reference range of 0.5 to 29 mU/L was determined for thyrotropin. CONCLUSION This study extends information on thyroid function of preterm infants and establishes reference ranges for this population.

Age-related changes in serum free thyroxine during childhood and adolescence

The purposes of this study were to reevaluate age-related changes in the concentration of serum free thyroxine (T4) between early infancy and adulthood, and to study the relationship of serum thyrotropin to these changes in free T4. Five hundred thirty-six healthy infants, children, adolescents, and adults between 1 day and 45 years of age were studied. Serum free T4 was determined by direct equilibrium dialysis, thyrotropin by a sensitive immunoassay, and total T4 by radioimmunoassay. Free T4 concentrations were age dependent and differed significantly (p < 0.001) at 1 to 4 days, 2 to 20 weeks, 5 to 24 months, 2 to 7 years, 8 to 20 years, and 21 to 45 years of age. Corresponding free T4 concentrations (mean +/- SEM) were 48.1 +/- 1.5, 20.3 +/- 0.6, 17.0 +/- 0.4, 19.9 +/- 0.4, and 21.2 +/- 0.4 pmol/L (3.74 +/- 0.12, 1.58 +/- 0.05, 1.32 +/- 0.03, 1.55 +/- 0.03, 1.32 +/- 0.03, and 1.65 +/- 0.03 ng/dl), respectively. Age-related reference ranges were determined. Changes in the relationship between serum thyrotropin and free T4 were complex, indicating age-related changes in pituitary thyroid regulation. No correlation existed between concentrations of free T4 and total T4 after the first 13 days of life (r2 < 0.01). We conclude that (1) pediatric free T4 concentrations and free T4-thyrotropin relationships differ from those in adults and (2) age-related changes in total T4 do not indicate age-related changes in either free T4 or thyroid function, after 13 days of age.

The Chloride-Phosphate Ratio in Hypercalcemia

Abstract Serum chloride and phosphate concentrations were measured in 52 hypercalcemic patients. The chloride values were higher (mean, 107 meq/litre) and phosphate, lower (mean, 2.6 mg/100 ml) in ...

Shared neuroendocrine patterns of post-traumatic stress disorder and alexithymia

&NA; High norepinephrine/cortisol ratios have been shown to be useful indicators of post‐traumatic stress disorder (PTSD). Alexithymia can result from overwhelming stress; thus, we hypothesized that sympathetic‐adrenal medullary/hypothalamic‐pituitary adrenal ratios would be positively associated with alexithymia severity. In the present study, we correlated 3‐methoxy‐4‐hydroxyphenylethylene glycol (MHPG)/adrenocorticotropic hormone (ACTH) and MHPG/cortisol ratios with self‐report Toronto Alexithymia Scale (TAS) scores in a group (n = 17) of nondepressed, formerly alcohol‐dependent men. The correlations between the respective ratios and TAS scores were 0.515 (p = 0.034) and 0.561 (p = 0.019). We suggest that increasing degrees of alexithymia are accompanied by an increasing separation of these two endocrine systems and then speculate that this dissociation has an anatomical basis in the lateralization of emotions.

Analog-Based Free Testosterone Test Results Linked to Total Testosterone Concentrations, Not Free Testosterone Concentrations

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.

Serum free thyroxine concentration is not reduced in premature infants with respiratory distress syndrome

OBJECTIVE We used improved methods of assay to determine whether pituitary-thyroid function is altered in premature infants with respiratory distress syndrome (RDS) during the first week of postnatal life. METHODS Serum free thyroxine (T4) was measured by direct equilibrium dialysis, total thyroxine (TT4) by radioimmunoassay, and thyrotropin by a sensitive immunometric assay in 90 premature infants (45 healthy control subjects and 45 with RDS) during their first week of life after 25 to 30 weeks of gestation. Infants in the RDS group received exogenous surfactant therapy. RESULTS Free T4 and thyrotropin concentrations of infants were not significantly different between RDS and control groups. As expected, infants with RDS had significantly lower serum total T4 concentrations compared with control infants (p < 0.001). This difference was present even after stratification for gestational age (25- to 27-week group, p = 0.012; 28- to 30-week group, p = 0.002). Lower total T4 concentrations were attributable to lower T4 binding to serum proteins among infants with RDS compared with control subjects, especially in the 25- to 27-week gestation group (p = 0.0075). CONCLUSION These data indicate that pituitary-thyroid function is not altered in premature infants with RDS. The low total T4 state in these premature infants is attributable solely to reduced serum T4 binding, as is often seen in acute nonthyroidal illnesses.

Counterpoint: Legitimate and Illegitimate Tests of Free-Analyte Assay Function: We Need to Identify the Factors That Influence Free-Analyte Assay Results

There have been, and continue to be, reports of discordant free thyroxine (T4)1 measurements when different free-T4 methods are applied to the same serum samples (1)(2)(3)(4). Our laboratory has attempted to identify some of the factors that contribute to these disparities. There are 4 separate components of serum T4: free T4, albumin-bound T4, transthyretin-bound T4, and thyroxine-binding globulin (TBG)-bound T4. Free T4 is the one component that moves across semipermeable membranes during equilibrium dialysis and ultrafiltration. As Midgley and Christofides point out, the gold standard free-T4 methods are designed to measure this component. We applied analog-type methods and an equilibrium-dialysis/immunoassay gold standard method to carefully prepared solutions in which we varied these 4 components of serum T4. This equilibrium-dialysis method agrees closely with a liquid chromatography–tandem mass spectrometry method applied to serum ultrafiltrates (4). We also applied this equilibrium dialysis/immunoassay and a group of 7 analog-type free-T4 assays to solutions of T4 that did not include T4-binding proteins (5); that is, the only form of T4 was free T4. The analog-type assays required 28- to 588-fold higher concentrations of free T4 than …

Assessing thyroid hormone status in a patient with thyroid disease and renal failure: from theory to practice.

A 35-year-old Asian male, treated for hyperthyroidism, systemic lupus erythematosis, and uremia presented with low serum total thyroxine (T4) and normal serum thyrotropin (TSH) levels. He had been receiving prednisone and methimazole for 15 weeks. Free T4 measured by direct equilibrium dialysis was in the hypothyroid range (0.3 ng/dL; normal, 0.8-2.7). Two possibilities were considered: (1) a weakly bound dialyzable inhibitor in uremic serum that interfered with this serum free T4 determination or (2) hypothyroidism with persistent TSH suppression because of prior hyperthyroidism. To determine whether a weakly bound inhibitor was involved, the patients serum was serially diluted using two diluents: (1) an ultrafiltrate of the patients serum, which would contain any unbound inhibitor, as well as free T4 and (2) an inert diluent. Free T4 measurements were similar with both, providing evidence against the presence of a dialyzable and ultrafilterable inhibitor. In conclusion, this patient was hypothyroid because of antithyroid drug administration, associated with prolonged central TSH suppression from preexisting hyperthyroidism. Discontinuation of methimazole resulted in normalization of serum total T4 and TSH values. Thus, paired, serial serum dilutions, using two different diluents, provided evidence for differentiation of appropriately low free T4 measurements (because of hypothyroidism), from spuriously low free T4 measurements (because of an interfering inhibitor).

Incidence of Low Free T4 Values in Premature Infants as Determined by Direct Equilibrium Dialysis

BACKGROUND: The incidence of transient reductions in serum free T4 (FT4) in premature infants may be overestimated because certain FT4 analytical methods underestimate FT4 concentrations. Transient reductions of FT4 measurements have been reported in the majority of premature newborn infants. Direct equilibrium dialysis (DED) does not underestimate FT4 concentrations and is the best available technique to measure serum FT4 in the premature infant.OBJECTIVE: To evaluate the incidence of low FT4 concentrations in premature infants using DED to measure FT4.DESIGN/METHOD: We measured FT4 by DED in infants with birth weight <1500 g. Infants were excluded if the following conditions were present: congenital anomalies or maternal thyroid disorders. Free T4 was measured at 14 days of life. Low FT4 was defined using a statistical definition of FT4 measurements <10.3 pmol/l (0.8 ng/dl).RESULTS: Free T4 was measured by DED in 114 infants. Low FT4 levels were seen in nine infants (7.9%).CONCLUSION: The incidence of low FT4 was much lower than previously reported when FT4 was measured using DED indicating that methodological issues are involved in the variability among estimates of the frequency of transient reduction in FT4.

Amikacin: a Rapid and Sensitive Radioimmunoassay

A rapid, sensitive, and specific radioimmunoassay has been developed for the measurement of amikacin, a new semisynthetic aminoglycoside antibiotic. Antisera were produced by immunizing rabbits with amikacin conjugated to porcine thyroglobulin. Sera were screened for amikacin antibodies with 3H-labeled amikacin. Free and bound radioactivity were separated by the second antibody method. As little as 5 ng of the antibiotic can be measured in serum, cerebrospinal fluid, and urine. The standard curve is unaffected by pH (5–11), ionic strength (0.01 M to 0.3 M), MgCl2 or CaCl2 (1 to 8 mM/liter), or urea (10–300 mg/100 ml). The antibodies raised to amikacin are not able to distinguish between kanamycin and amikacin. The cross reactivity of all other antibiotics tested was <0.1%. Co-incubation of first and second antibody allows for a 30-min incubation period, making the assay useful for stat determinations. The standard curve is linear on a logit-log plot, allowing for rapid computer analysis of the standard curve, interpolation of sample values, and quality-control monitoring.