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PubMed1993

Free thyroxine assessed with three assays in sera of patients with nonthyroidal illness and of subjects with abnormal concentrations of thyroxine-binding proteins.

Three methods for estimating free thyroxine (FT4) in serum were studied: equilibrium dialysis, the SPAC-ET FT4 radioimmunoassay kit, and the Amerlite MAB FT4 luminometric assay. Serum samples from 10 subjects with above-normal thyroxine-binding globulin (TBG), 6 with low TBG, 30 with familial dysalbuminemic hyperthyroxinemia (FDH), 13 with nonesterified fatty acids (NEFA) concentrations in serum > 1.0 mmol/L, and 178 patients with various degrees of nonthyroidal illness (NTI) were measured and compared with samples from 42 euthyroid blood donors. The Amerlite MAB FT4 assay compared well with equilibrium dialysis, whereas the SPAC-ET assay averaged 40% lower. All three assays were not influenced by changes in TBG and showed no or only little changes in the presence of NEFA. Mean FT4 values in the FDH samples were somewhat higher than in controls when measured with the SPAC-ET assay, about equal with equilibrium dialysis, and somewhat below the mean control value with the Amerlite MAB FT4 assay, although individual results were within the control reference range. In NTI patients, no FT4 values were below the control reference range by the Amerlite MAB FT4 assay, 4 of 178 were below this range by equilibrium dialysis, and 1 of 178 was below this range by the SPAC-ET assay. In all assays a large proportion of the NTI samples showed FT4 values above the control reference range, a result that will interfere with the efficacy of these assays for assessing thyroid function in NTI patients.

Dialysis↗
PubMed1988

Thyroxine uptake by human hepatoma cells from serum of patients submitted to long-term thyroxine suppressive therapy.

The significance of thyroxine (T4) uptake from serum in the assessment of thyroid status was evaluated, using human hepatoma (Hep G2) cells, in 30 euthyroid subjects, 6 hypothyroid and 19 hyperthyroid patients, and in 23 athyreotic cancer patients under T4 suppressive therapy. Cellular thyroxine (CT4) was determined according to Sarne and Refetoff, J. Clin. Endocrinol. Metab. 61: 1046, 1985. CT4 averaged 9.9 +/- 2.8 pg/well (mean +/- SD, range 5.7-15.3) in euthyroid subjects, 1.5 +/- 1.0 pg/well (range 0.05-4.2) in hypothyroid patients, 40.5 +/- 18.8 pg/well (range 18.3 +/- 104.7) in hyperthyroid patients, and 23.7 +/- 7.2 pg/well (range 14.2-40.2) in T4-treated patients. In eu-, hypo- and hyperthyroid patients, a significant correlation was observed between CT4 and free T4 index (FT4I), free T4 (FT4) or Sex Hormone Binding Globulin (SHBG) values. In T4-treated patients, CT4 values were correlated with FT4I values, but not with FT4 or SHBG levels. All T4-treated patients with elevated SHBG levels had elevated FT4, FT4I and CT4 values. In contrast, of the 16 T4-treated subjects with normal serum SHBG concentrations, all but one had normal FT3, 3 (19%) had elevated FT4, 10 (62%) elevated FT4I and 13 (81%) elevated CT4, but all (100%) had undetectable TSH levels. Thus, considering serum SHBG concentrations as a parameter of hepatic tissue response to thyroid hormone, CT4 values, at least in T4-treated patients, do not accurately reflect the liver responsiveness to thyroid hormone action.

Carcinoma, Hepatocellular↗
PubMed1987

Relationship between effects of added albumin, initial free thyroxine value and endogenous serum-binding protein concentrations on Amerlex free thyroxine estimations.

We studied the effect of adding purified human albumin to sera on free thyroxine (FT4) values obtained with Amerlex radioimmunoassays. Apparent FT4 values increased with progressive addition of albumin in vitro. The effect was smallest with low and greatest with high initial FT4 concentrations, which were also linearly correlated with the incremental increase in FT4 values per g/l albumin added. Wide variations in either endogenous thyroxine binding globulin (TBG) or albumin concentrations in patient serum had little effect on the rate of increase in FT4 values when albumin was added in vitro. From Mass Action theory, calculations of the binding affinity of the endogenous albumin for the analog (2.1 X 10(5) l/mol) gave values nearly half that of the added albumin (3.94 X 10(5) l/mol). Distortions in Amerlex FT4 values caused by adding albumin in vitro may exaggerate its importance as a tracer binder and such results may be unrepresentative of patient samples.

Albumins↗
PubMed1983

Frequency of euthyroid sick syndrome as assessed by free thyroxine index and a direct free thyroxine assay. A limitation of FT4 assays.

The frequency of the euthyroid sick syndrome (ESS) was assessed in 3444 hospital patients. For the first 6 months the free thyroxine index (FTI) was used: 3.6% of 1674 patients had ESS compared with 17.3% of 1770 in the second 6 months diagnosed by a direct free thyroxine (FT4) assay ("Amerlex'). The relative frequencies of each type of ESS (low triiodothyronine [T3] only, low T3 plus low FT4 or FTI, and high FT4 or FTI with normal or low T3) and the frequencies of hypothyroidism (high TSH) and hyperthyroidism (high FTI/FT4 and T3) were similar in both periods. In the FT4 period requests for T3 assays increased by 300%. Measurements of both FT4 and FTI in 46 patients with ESS confirmed that ESS is diagnosed more frequently with the FT4 assay: the distribution of normal and abnormal T3 and reverse T3 levels were similar in FT4 ESS and FTIESS. FT4 is labile in illness and thus direct FT4 measurements also fluctuate, not necessarily reflecting thyroid status.

Diagnosis, Differential↗
PubMed1984

L-thyroxine contamination of pharmaceutical D-thyroxine: probable cause of therapeutic effect.

Studies have shown that pharmaceutic preparations of the stereo isomers of thyroxine differ with respect to thyromimetic potency and lipid level-lowering effects. We applied a stereospecific assay for dextrothyroxine (DT4) and levothyroxine (LT4) to determine whether the biologic effects observed after the administration of DT4 (Choloxin; Flint Laboratories) resulted from inherent biologic activity of DT4, conversion of DT4 to LT4 in vivo, or LT4 contamination of Choloxin tablets. Choloxin was administered in a dose of 8 mg/day for 5 mo to nine athyreotic subjects who were then treated with pharmaceutic LT4 (Synthroid), 0.2 mg/day for an additional 5 mo. Analysis showed that LT4 contamination of Choloxin tablets ranged from 0.50% to 2.30%. This degree of contamination resulted in physiologically significant doses of LT4 in the 8 mg/day doses of Choloxin. During the treatment with two different lots of Choloxin, serum LT4 accounted for 33% to 53% of the measurable serum total thyroxine. The degree of LT4 contamination in Choloxin tablets was sufficient to account for the observed serum LT4 levels and casts doubt on the conclusions derived from previous studies in which Choloxin was used as the source of "DT4."

Adolescent↗
PubMed1987

Studies on in-vivo capacity of thyroxine deiodinating system in rat liver: biliary excretion of several iodothyronines after increasing loading doses of thyroxine.

Eighteen male rats were inserted with polyethylene tubings into bile duct and femoral vein and two rats each were injected a series of doses from 5 to 1280 micrograms L-thyroxine into a venous cannula. The bile was collected for three subsequent 2-hr periods and the excretion of total thyroxine (T4), triiodothyronines (T3 and rT3) and all diiodothyronines (3,3'-T2, 3,5-T2 and 3',5'-T2) was estimated as well as that of conjugated T4 and T3. The excretion of all compounds was considerably increased as early as within the first 2-hr period. Almost linear dose-response relationship was found between the dose of T4 and its biliary excretion up to the dose of 640 micrograms, only smaller increase being observed after 1280 micrograms T4. Similar relationship was found also in the excretion of diiodothyronines, while that of triiodothyronines after 1280 micrograms T4 was slightly less than after 640 micrograms T4. The excretion of rT3 was consistently about twice as high as that of T3. The data on diiodothyronine excretion suggested a preferential conversion of rT3 to 3',5'-T2 and that of T3 to 3,5-T2 over that to 3,3'-T2. The ratio of 3,5-T2/T3 after different doses of T4 was about 2--4 times higher than that of 3',5'-T2/rT3 suggesting the higher deiodination rate of T3 than that of rT3.

Animals↗
PubMed1989

A study of hepatic metabolism of thyroxine in BB/W rats treated with L-thyroxine.

The effect of thyroxine (T4) on T4 conversion to triiodothyronine (T3) and reverse T3 (rT3) was studied in BB/W rats. A colony of 38 BB/W rats was obtained and half were treated with thyroxine (T4), 1 mg per liter of drinking water. At 106 days of age the following groups were identified: nondiabetic, no T4 treatment, 8 rats; nondiabetic, T4 treated, 8 rats; diabetic, no T4 treatment, 10 rats; diabetic, T4 treated, 7 rats. All animals with diabetes were treated with insulin. T4 conversion to T3 and rT3 was assessed in liver homogenates in 0.1 M Tris-HCl buffer, pH 7.4, with or without 5 mM dithiothreitol (DDT). Serum T4 and rT3 were significantly elevated in both T4-treated groups (P less than 0.001), while serum T3 was not affected in either. Basal T4 deiodination to T3 by the liver homogenate did not change on treatment with T4; the addition of DTT increased T3 production in the homogenate from T4 treated nondiabetic animals (P less than 0.05). In both nondiabetic and insulin-treated diabetic rats there was no effect of T4 on the rate of rT3 production. Since, in the rat, 30-40% of circulating T3 is a direct contribution of thyroid gland secretion, and that would be absent in our T4-suppressed animals, the normal serum T3 may reflect increased absolute peripheral T3 production from the greater concentration of circulating T4.

Animals↗
PubMed1999

Serum thyroxine binding capacity-dependent bias in an automated free thyroxine assay.

The magnitude of serum thyroxine (T4) binding capacity (sBC) dependent bias in the AXSYM free thyroxine (FT4) assay was assessed using two recently described tests. One of the tests uses a direct equilibrium dialysis (ED) FT4 assay as the reference method. The results obtained with the AXSYM method were compared with those obtained by the ED FT4 method in patient sera having a wide range of sBC. The other test involves comparison of the FT4 results obtained following dilution of sera by an inert buffer, to theoretically derived FT4 results. As serum dilution causes a predictable decrease in sBC, the demonstration of a negative bias whose magnitude increases in parallel to the dilution, is indicative of an sBC-dependent bias. The AXSYM FT4 assay exhibited a significant sBC-dependent bias. This sBC-dependent bias is likely to have been caused by the presence of significant amounts of T4 binding proteins in the assay reagents.

Dialysis↗
PubMed2002

Free thyroxine solid-analog immunoassays. investigation of the albumin effect on the antibody binding to immobilized thyroxine-protein conjugates.

A reliable one-step, analog-based FT4 immunoassay should be characterized by minimal binding of the serum thyroxine-binding proteins to thyroxine-analogs, used either as liquid-phase tracers or as solid-phase reagents. In this work, we investigated the effect of serum albumin concentration on the anti-T4 antibody binding to immobilized T4-protein conjugates with respect to the molecular weight (MW) and the T4-to-protein molar ratio of the conjugates. It was found that the presence of albumin in the serum sample, at concentrations up to 120 g/L, slightly decreased the antibody binding to the immobilized conjugates (less than 10%). In addition, this decrease was independent of both the MW and the T4-to-protein molar ratio of the conjugates. On the other hand, using the same conjugates as liquid-phase labeled analogs, the observed decrease ranged between 2 and 33%, depending on the MW of the conjugate. These findings indicate that the use of immobilized T4-protein conjugates may improve FT4 immunoassay reliability, concerning, especially, the issues of serum albumin concentration and/or affinity alterations.

Humans↗
PubMedDate not supplied

Duration of effects of phenylbutazone on serum total thyroxine and free thyroxine concentrations in horses.

The objectives of this study were to determine if phenylbutazone decreased serum thyroxine (TT4) and free thyroxine (FT4) concentrations using radioimmunoassay and equilibrium dialysis techniques in horses, and, if so, an additional objective was to determine the duration of this decreased concentration once phenylbutazone administration was discontinued. Serum TT4 and FT4 concentrations were determined before and after administration of 4.4 mg/kg of phenylbutazone i.v. bid for 5 days. Treatment with phenylbutazone caused a significant decrease in TT4 and FT4 concentrations (P < .05). Serum TT4 concentration significantly decreased after day 4 of treatment and remained significantly below baseline value for 10 days after discontinuing phenylbutazone administration; it returned to a value not different from the baseline value by the 11th day. Serum FT4 concentration significantly decreased after day 4 of treatment and remained significantly below the baseline value for only 1 day after phenylbutazone administration was discontinued; it returned to a value not different from the baseline value by the 3rd day after discontinuation of phenylbutazone. These results indicate that serum TT4 and FT4 should not be used to evaluate thyroid function in horses receiving phenylbutazone. In addition, results should be interpreted cautiously when phenylbutazone has been administered within 2 days (for FT4) or within 10 days (for TT4) of sample collection.

Animals↗
PubMed2000

Usefulness of thyroxine and free thyroxine filter paper measurements in neonatal screening for congenital hypothyroidism of preterm babies.

INTRODUCTION: Low thyroxine (T4) with normal thyroid stimulating hormone (TSH) is a well known condition in preterm (PT) infants. The establishment of T4 and freeT4 (FT4) values in filter paper dried blood spots in PT could provide useful information in the neonatal period. OBJECTIVE: To study T4 and FT4 levels in dried blood filter paper samples of PT and full term (FT) babies. METHODS: We measured T4 by fluoroimmunoassay (FIA) DELFIA and TSH by IFMA DELFIA (Wallac Inc Turku, Finland) in 193 PT (26 to 37 weeks of gestational age (GA)) in samples from the first and second week of life and in 153 FT babies in the first week of life. In 131 PT and 31 FT we determined FT4 in filter paper blood spots using FIA (Alonso Fernandez J). Infants were grouped according to GA. RESULTS: There was a significant difference in T4 between PT and FT (p < 0.001). The lowest T4 levels were at 26 to 29 weeks GA. T4 values were lower in the second week. FT4 in PT up to 35 weeks GA, during the first week, was significantly different with FT infants (G1 to G3 p < 0.01, G4 p < 0.05). FT4 values in the first and second weeks of life did not vary. CONCLUSIONS: T4 values were significantly lower in PT than in FT neonates, increasing with GA. PT infants had low T4 with normal FT4 values. This could suggest a decreased thyroxine binding globulin (TBG) or decreased protein binding and/or an adaptative mechanism that would not require therapeutical intervention.

Congenital Hypothyroidism↗
PubMed1986

Primary hypothyroidism with paradoxically elevated plasma thyroxine due to thyroxine-binding antibody.

Investigation of an elderly patient with clinical hypothyroidism revealed a low serum triiodothyronine (T3) but markedly raised levels of total thyroxine (TT4) and free thyroxine (fT4) when measured by radio-immunoassay (RIA). Primary hypothyroidism was suggested by an elevated basal thyroid stimulating hormone (TSH) and confirmed by a low fT4 when measured by a commercial microencapsulated antibody method. The paradoxically elevated levels of fT4 and TT4 were due to the presence of an antibody which specifically bound T4 and grossly interfered with the RIA thus complicating the diagnosis and later the assessment of replacement therapy. The sensitive immunoradiometric TSH assay proved of value in resolving these problems.

Aged↗
PubMed1976

Measurements of free thyroxine: comparison of per cent of free thyroxine in diluted and undiluted sera.

The effect of dilution on values of the per cent of free T4 in serum (%FT4) measured by equilibrium dialysis, was determined by comparing the values for %FT4 in diluted and undiluted specimens. The specimens studied were sera obtained from 10 normal subjects, 10 patients with hyperthyroidism, 5 with hypothyroidism, and 9 women with normal pregnancy. The method of analysis employed was that described by Sterling and Brenner, except that the 125I-thyroxine used was predialyzed to remove non-thyroxine contaminants and that "corrected" values for %FT4 were calculated by dividing measured values by the numerical extent to which the serum had been diluted.

Female↗
PubMed1997

Increased urinary thyroxine sulfate excretion in thyroxine therapy.

Although increased thyroxine sulfate (T4S) levels have recently been detected in fetal serum and amniotic fluid, changes in patients in a high thyroxine (T4) state remain unclarified. This study was conducted to determine the changes in T4S in thyroid hormone regulation in women receiving suppressive T4 therapy. With a highly sensitive and specific radioimmunoassay, we measured the serum and urinary concentrations of T4S in 16 premenopausal women with benign nodular goiter before and after three months administration of T4 (3.2 micrograms/kg/day). Serum levels of other thyroid hormones were also measured. Significant increases in mean serum T4 levels post-treatment (11.1 vs. 6.6 micrograms/dL pre-treatment; P < 0.01) were found, although only low T4S levels were detectable in serum both pre- and post-T4 treatment. The mean urinary or creatinine corrected urinary T4S values post-treatment were significantly increased (20 ng/dL or 396 ng/g creatinine vs. 12 ng/dL or 174 ng/g creatinine pre-treatment, P < 0.01). There was a significant correlation between increased creatinine-corrected urine T4S and increased serum free T4. Our results indicate that the sulfation of T4 may be related to the regulation of thyroid hormone metabolism in T4-treated subjects with relative hyperthyroxinemia.

Adult↗
PubMed1978

Low serum thyroxine due to anti-thyroxine antibody.

In one case of untreated Hashimoto's thyroiditis, the serum thyroxine (T4) value, obtained by radioimmunoassay (RIA) employing resin to separate bound and free T4, was significantly lower than that obtained by competitive protein binding analysis (CPBA). The discrepancy was found to be due to the presence of anti-thyroxine autoantibody in the serum. This phenomenon is considered to be of practical importance in interpreting the T4 value by RIA in cases with autoimmune thyroid diseases.

Antibodies↗
PubMed2006

Effects of deracoxib and aspirin on serum concentrations of thyroxine, 3,5,3'-triiodothyronine, free thyroxine, and thyroid-stimulating hormone in healthy dogs.

OBJECTIVE: To evaluate the effects of deracoxib and aspirin on serum concentrations of thyroxine (T4), 3,5,3'-triiodothyronine (T3), free thyroxine (fT4), and thyroid-stimulating hormone (TSH) in healthy dogs. ANIMALS: 24 dogs. PROCEDURE: Dogs were allocated to 1 of 3 groups of 8 dogs each. Dogs received the vehicle used for deracoxib tablets (PO, q 8 h; placebo), aspirin (23 to 25 mg/kg, PO, q 8 h), or deracoxib (1.25 to 1.8 mg/kg, PO, q 24 h) and placebo (PO, q 8 h) for 28 days. Measurement of serum concentrations of T4, T3, fT4, and TSH were performed 7 days before treatment (day -7), on days 14 and 28 of treatment, and 14 days after treatment was discontinued. Plasma total protein, albumin, and globulin concentrations were measured on days -7 and 28. RESULTS: Mean serum T4, fT4, and T3 concentrations decreased significantly from baseline on days 14 and 28 of treatment in dogs receiving aspirin, compared with those receiving placebo. Mean plasma total protein, albumin, and globulin concentrations on day 28 decreased significantly in dogs receiving aspirin, compared with those receiving placebo. Fourteen days after administration of aspirin was stopped, differences in hormone concentrations were no longer significant. Differences in serum TSH or the free fraction of T4 were not detected at any time. No significant difference in any of the analytes was detected at any time in dogs treated with deracoxib. CONCLUSIONS AND CLINICAL RELEVANCE: Aspirin had substantial suppressive effects on thyroid hormone concentrations in dogs. Treatment with high dosages of aspirin, but not deracoxib, should be discontinued prior to evaluation of thyroid function.

Administration, Oral↗
PubMed2006

Circadian variations of serum thyroxine, free thyroxine and 3,5,3'triiodothyronine concentrations in healthy dogs.

This study was to determine the daily fluctuation of serum thyroxine (tT4), free thyroxine (fT4), 3,5,3'-triiodothyronine (T3) concentrations in healthy dogs. Thyroid function of these dogs was evaluated on the basis of results of TSH response test. Samples for the measurement of serum tT4, fT4, and T3 concentrations were obtained at 3- hour intervals from 8 : 00 to 20 : 00. Serum tT4, fT4, and T3 concentrations were measured by the enzyme chemiluminescent immunoassay (ECLIA). Mean T3 concentrations had no significant differences according to the sample collection time during the day. Mean tT4 and fT4 concentrations at 11 : 00 were 3.28 +/- 0.86 microg/dl and 1.30 +/- 0.37 ng/dl, respectively and mean tT4 and fT(4) at 14:00 were 3.54 +/- 1.15 microg/dl and 1.35 +/- 0.12 ng/dl, respectively. These concentrations were significantly high compared with tT4 and fT4 concentrations at 8:00, which were 1.75 +/- 0.75 microg/dl and 0.97 +/- 0.25 ng/dl, respectively (p < 0.05). According to the sample collection time, mean tT4 and fT4 concentrations changed with similar fluctuation during the day. Based on these results, it was considered that measurement of serum tT4 and fT4 concentrations from 11 : 00 to 14 : 00 might more easily diagnose the canine hypothyroidism in practice.

Animals↗
PubMed2003

Effect of dosing and sampling time on serum thyroxine, free thyroxine, and thyrotropin concentrations in dogs following multidose etodolac administration.

Thirty-eight dogs with orthopedic disorders received etodolac, an NSAID, at 10.0 to 13.3 mg/kg PO once daily for 14 to 19 days. Mean total thyroxine (T4), free thyroxine (fT4), and canine thyrotropin (cTSH) values before and after etodolac administration were compared using paired t-tests. A significant (P <.05) decrease in T4 values occurred after etodolac administration with 21% of these values falling below the reference range. A significant (P <.05) increase in cTSH following etodolac administration, but none of the values was above the reference range. No significant changes occurred in mean fT4 values; however, 10% of the values fell below the reference range. In conclusion, T4 and fT4 test results should be interpreted with caution in dogs receiving etodolac.

Animals↗