Contribution of methodological artifacts to the measurement of T3 concentration in serum

Isolation of labeled triiodothyronine from serum using affinity chromatography: application to the extimation of the peripheral T4 to T3 conversion in rats

A method for the isolation of small quantities of labeled 3,5,3' -triiodothyronine (T3) from serum or thyroid extracts is described. Conjugates of rabbit anti-T3 antibody to Sepharose 4B are incubated with 0.5 to 1 ml of human or rat serum at pH 8.6 for 1 hr. The tubes are centrifuged and washed with buffer followed by 6 M guanidine to remove nonspecifically bound labeled thyroxine (T4). The fraction of T3 and T4 bound to the Sepharose conjugate varies depending on the concentration of serum in the initial incubation tubes, the T3 and T4 content, and the specificity of the antiserum used. In a system that contains 0.5 ml of normal human serum, 1 ml of glycine-acetate buffer (pH 8.6), and 0.25 ml settled Sepharose-anti-T3 conjugate, the T3 to T4 binding ratio was generally 150-200, with up to as much as 50% of T3 bound to the pellet. The coefficient of variation of the method is less than 5%, and it may be performed in a matter of hours. There is no detectable conversion of T4 to T3 during the separation process. Using this technique, conversion of T4 to T3 was evaluated in euthyroid rats after injection of 125l-T4. Over the period of 36-72 hr after injection, a ratio of T3 to T4 of 0.74 +- 0.06 x 10-2 (mean +- SE) was present in the plasma. Using the calculated metabolic clearance rates for T3 and T4 in these animals, fractional conversion of T4 to T3 was estimated to be 27%, in good agreement with results obtained by other techniques. This method would appear to be of value for specific isolation of the small quantities of T3 produced from T4 after in vivo or in vitro T4 to T3 conversion.

Radioimmunoassay and the hormones of thyroid function

Radioimmunoassay (RIA) has provided the tools for wide-reaching investigations that have changed and continue to change many important concepts of thyroid physiology and pathosphysiology. The RIA vor human thyrotropin (TSH) was developed in 1965; development of the RIA for triiodothyronine (T3), thyroxine (T4), thyroxine-binding globulin (TBG), and, recently, thyrothropin-releasing hormone (TRH) and thyroglobulin (Tg) followed. The capacity to measure nanogram and picogram concentrations with relative ease and speed has permitted the demonstration of dynamic relationships of the intrathyroidal and circulating thyroid hormones to each other and to the pituitary and hypothalamic regulating hormones. Evidence for the presence of cross-influences between TRH and other hypothalamic regulating hormones on the secretion of pituitary hormones has accumulated. The impact of the new information on clinical practice is now becoming evident. There is new appreciation of the value of assaying serum T3 and TSH concentrations in the clinical management of patients with disturbed function of the thyroid, pituitary, or hypothalamus. The necessary components for RIA performance can be purchansed separately or in kit form from commercial sources. With appropriate quality-control procedures, precise, sensitive, and reliable data can be generated. Awareness of the specific technical problems relating to the RIA of these hormones is absolutely necessary to assure reliable results. The availability of kits or their components permits the performance of these studies in the community hospital and in reliable commercial-service laboratories.

Estimation of thyroxine and triiodothyronine distribution and of the conversion rate of thyroxine to triiodothyronine in man

Studies on peripheral metabolism of simultaneously administered 125-I-labeled L-thyroxine ([125-I]T4) and 131-I labeled L-trilodothyronine ([131-I]T3) were performed in five normal subjects, in four patients with untreated hypothyroidism, and in 3 hypothyroid patients made euthyroid by the administration of T4. The fractional turnover rate (lambda 03) of thyroid hormones irreversibly leaving the site of degradation and the volumes of pool 1 (serum V1) of pool (interstitial fluid, V2), and of pool 3 (all tissues, V3)were obtained by using a three-compartment analysis. In addition to the turnover studies, the ratios for the in vivo T4 to T3 conversion were determined by paper chromatographic study in sera obtained 4, 7, and 10 daysafter the injection. The rate (K12) of the extrathyroidal conversion of T4 to T3 was also estimated by the compartment analysis. The T3 distribution volume (V3) of pool 3, in which T3 is utilized and degraded, was about 60% of totaldistribution volume (V=V1+V2+V3) in normal subjects, whereas only about 25% of the extrathyroidal T4 pool was in the intracellular compartment, indicating that T3 is predominantly an intracellular hormone..

Serum triiodothyronine and thyroxine in the neonate and the acute increases in these hormones following delivery

Low triiodothyronine (T(3)) and high normal thyroxine (T(4)) concentrations are present in cord sera from full term infants. To examine this phenomenon further, radioimmunoassay of T(3) and T(4) was carried out in paired maternal and cord sera as well as capillary sera from neonates at different intervals after delivery. Free T(3) and free T(4) concentrations were also estiamted in cord and maternal sera by equilibrium dialysis. In 12 paired specimens, the T(3) concentration in cord sera was significantly lower than the maternal level (51+/-4 vs. 161+/-11 ng/100 ml, mean +/-SE). Mean free T(3) concentration was also lower in the cord samples (0.15+/-0.02 vs. 0.31+/-0.04 ng/100 ml). whereas total and free T(4) concentrations were not significantly different. Umbilical vein and artery samples from 11 neonates did not differ significantly in their T(3) and T(4) concentrations. In seven infants the mean T(3) concentration increased from 51+/-3 ng/100 ml at delivery to 79+/-13 at 15 min and 191+/-16 at 90 min. In four other infants the mean T(3) concentration at 24 and 48 h was not significantly different from the 90 min value of the previous group. Less pronounced changes were observed for T(4) which increased from 12.3+/-2.0 mug/100 ml (mean +/-SE) at delivery to 14.1+/-1.9 at 90 min and appeared to have reached a plateau at approximately twice the cord value by 24-48 h after delivery.The maternal-fetal gradient observed for free T(3) is further evidence of the autonomy of the fetal thyroidpituitary axis. The time course of the abrupt increase in serum T(3) in the neonate suggests that it results from the earlier acute increase in serum TSH which occurs shortly after birth. This suggests that the neonatal thyroid contains significant quantities of T(3). Therefore, unavailability of thyroidal T(3) does not appear to explain the low total and free T(3) concentrations present in the sera of newborns.

A new radioimmunoassay for plasma L-triiodothyronine: measurements in thyroid disease and in patients maintained on hormonal replacement

A new procedure for the radioimmunoassay of l-triiodothyronine (T(3)) in human plasma is described in which the iodothyronines are separated from the plasma proteins before incubation with a specific antiserum to T(3). The antibody bound and free T(3) are separated with dextran-coated charcoal. In this system, the mean recovery of T(3) added to plasma was 97.9% and both in vitro conversion of l-thyroxine (T(4)) to T(3) and cross-reaction between T(4) and the anti-T(3) antibody were undetectable (less than 0.1%). The assay procedure allowed the measurement of T(3) in up to 0.5 ml of plasma resulting in improved assay sensitivity (6 ng/100 ml). The mean plasma T(3) in normal subjects was 146+/-24 ng/100 ml (sd). Mean T(3) concentration was increased in hyperthyroidism (665+/-289 ng/100 ml) and decreased in hypothyroidism (44+/-26 ng/100 ml). In patients with severe hypothyroidism, plasma T(3) was between 7 and 30 ng/100 ml. Plasma T(3) concentration was relatively constant throughout the day in three euthyroid subjects. In contrast, in hypothyroid subjects on replacement therapy with T(3), a T(4): T(3) combination or desiccated thyroid plasma T(3) was markedly elevated for several hours after ingestion of the medication. Plasma T(3) was unchanged throughout the day in patients treated with T(4). Thus, insofar as plasma T(3) levels are concerned, replacement therapy with T(4) appears to mimic the euthyroid state more closely than other preparations.

Direct immunoassay of triiodothyronine in human serum

A sensitive and precise radioimmunoassay for the direct measurement of triiodothyronine (T(3)) in human serum has been designed using sodium salicylate to block T(3)-TBG binding. This assay is sufficiently sensitive to quantitate T(3) accurately in 50-100 mul of normal serum and to measure quantities as small as 12.5 pg in 0.2 ml of hypothyroid serum. The T(3) values observed in euthyroid subjects and in patients with various thyroid diseases are as follows: euthyroid (38) 1.10+/-0.25 (SD) ng/ml, hypothyroid (25) 0.39+/-0.21 (SD) ng/ml, and hyperthyroid (24) 5.46+/-4.42 (SD) ng/ml. The levels of T(3) parallel the thyroxine (T(4)) concentration in the sera of these subjects. In eight pregnant women at the time of delivery, T(3) concentrations were in the upper normal range (mean 1.33 ng/ml). The levels of T(3) in cord serum obtained at the time of delivery of these patients (mean 0.53 ng/ml) are distinctly lower and close to the hypothyroid mean. Administration of 10 U of bovine thyroid-stimulating hormone (TSH) to euthyroid subjects causes a two-fold increase in serum T(3) levels within 8 hr. At this time, the increase in serum T(4) concentration is only 41%. In two subjects in whom thyroid secretion was acutely inhibited, one after pituitary surgery and another after thyroidectomy, the serum T(3) fell into the hypothyroid range within 1-2 days. Thus, serum T(3) concentrations appear to be a sensitive index of acute changes in thyroid hormone secretion and should prove to be a useful adjunct to both the clinical and physiological evaluation of thyroid function.

Simultaneous measurement of thyroxine and triiodothyronine peripheral turnover kinetics in man

Serum triiodothyronine (T(3)) kinetics in man have been difficult to define presumably due to the interference of iodoproteins generated during the peripheral metabolism of T(3). The use, in the present study, of an anion-column chromatographic method for separation of serum T(3) as well as thyroxine (T(4)) from these iodoproteins has overcome this technical handicap. Simultaneous measurement of serum (125)I-T(3) and (131)I-T(4) kinetics were performed in 31 subjects from the clinical categories of euthyroid, primary hypothyroid, thyrotoxic and posttreatment hypothyroid Graves' disease, factitial thyrotoxic, and idiopathically high and low thyroxinebinding globulin states. The normal mean T(3) fractional turnover rate (kT(3)) was 0.68 (half-life = 1.0 days), increased in toxic Graves' disease patients to 1.10 (half-life = 0.63 days), and decreased in primary hypothyroid patients to 0.50 (half-life = 1.38 days). The mean T(3) equilibration time averaged 22 hr except in hypothyroid and high thyroxine-binding globulin (TBG) patients where the equilibration period was delayed by 10 hr. The mean T(3) distribution space in normal subjects was 38.4 liters. This was reduced in subjects with high TBG levels (26 liters) and increased in patients with low TBG and in all hyperthyroid states (53-55 liters). The normal serum T(3) concentration was estimated by radioimmunoassay to be 0.106 mug/100 ml. Combined with the mean T(3) clearance value of 26.1 liters/day, the calculated T(3) production rate was 27.6 mug/day. The mean T(3) production rate increased to 201 mug/day in thyrotoxic Graves' disease patients and was reduced to 7.6 mug/day in primary hypothyroid subjects. T(3) production rate was normal in subjects with altered TBG states. The ratio of T(3) to T(4) production rate in normal subjects was 0.31 and was unchanged in patients with altered TBG values. This ratio was increased in all Graves' disease patients with the highest value being 0.81 in the posttreatment hypothyroid Graves' disease group. This apparent preferential production of T(3) may have been responsible for the retention of rapid turnover kinetics for T(3) and T(4) observed in treated Graves' disease patients. The finding that factitial thyrotoxic patients also displayed similar rapid T(3) and T(4) turnover kinetics indicates that these alterations are not a unique feature of Graves' disease per se. When comparing the peripheral turnover values for T(3) and T(4) in man, it is apparent that alterations in metabolic status and serum TBG concentration influence both hormones in a parallel manner; however, changes in metabolic status seem to have a greater influence on T(3) kinetics while alterations in TBG concentrations have a greater effect on T(4). These observations probably relate to the differences in TBG binding affinity and peripheral tissue distribution of these two hormones.

Triiodothyronine radioimmunoassay

Highly specific antisera to triiodothyronine (T(3)) were prepared by immunization of rabbits with T(3)-bovine serum albumin conjugates. Antisera with T(3) binding capacity of up to 600 ng/ml were obtained. The ability of various thyronine derivatives to inhibit the binding of T(3-) (125)I to anti-T(3) serum was found to vary considerably. l-T(3), d-T(3) and several triiodoanalogues were potent inhibitors of the reaction. Little inhibition of T(3-) (125)I binding was produced by l-thyroxine (T(4)) or other tetraiodo- analogues, thyronine or iodotyrosines. Chromatography of several T(4) preparations indicated that most of their very slight activity could be ascribed to contamination with T(3). Successful assay of T(3) in serum was accomplished by the addition of diphenylhydantoin to the assay system. Under these circumstances, recovery of T(3) added to serum was excellent, and addition of T(4) was without significant effect. Serum T(3) concentrations in normal subjects averaged 145 +/-25 ng/100 ml (sd). Increased concentrations (429 +/-146 ng/100 ml) were observed in hyperthyroid patients whereas those with hypothyroidism had serum T(3) levels of 99 +/-24 ng/100 ml. Elevated T(3) concentrations were found also in hypothyroid patients receiving 25 mug or more of T(3) daily and in those receiving 300 mug of T(4) daily. Serial measurements of T(3) concentrations in subjects after oral T(3) administration revealed peak T(3) concentrations 2-4 hr after T(3) administration. Intramuscular administration of thyrotropin (TSH) resulted in earlier and more pronounced increases in serum T(3) than in serum T(4) concentrations. Triiodothyronine (T(3))(1) was recognized to be a biologically active secretory product of the thyroid gland over a decade ago (1). Recent studies have indicated that it is formed extrathyroidally as well (2, 3). Nevertheless, relatively little information concerning the role of T(3) secretion in different thyroid disorders has been accumulated until very recently. Methods for the measurement of T(3) which require its extraction from plasma, and often its separation from thyroxine as well, have been described by several investigators (4-11). These methods have proven useful, but they are relatively complicated, the number of samples that can be assayed is limited, and they may be affected by in vitro deiodination of thyroxine. More recently the radioimmunoassay technique has been applied to the measurement of T(3). Several preliminary reports have appeared describing the preparation of antibody to triiodothyronine by immunization of animals with T(3)-protein conjugates and its use for the measurement of T(3) in serum (12-15). The present report describes the development of a radioimmunoassay for the measurement of T(3), studies of the specificity of the anti-T(3) serum, and some initial studies which indicate that the method is applicable to the measurement of T(3) in unextracted serum.

Serum triiodothyronine: measurements in human serum by radioimmunoassay with corroboration by gas-liquid chromatography

Serum triiodothyronine (T(3)) has been measured by radioimmunoassay and corroborated by analysis of the identical samples with a previously described gas-liquid chromatographic technique. Special features of the radioimmunoassay procedure which permit determinations in unextracted serum include the use of a T(3)-free serum preparation for the construction of the standard curve and of tetrachlorothyronine to inhibit binding of T(3) to thyroxine-binding globulin.T(3) values by radioimmunoassay were 138 +/-23 ng/100 ml (mean +/-SD) in 82 normal subjects, 62 +/-9 ng/100 ml in 45 hypothyroid patients, and 494 +/-265 ng/100 ml in 60 patients with toxic diffuse goiter. In the hypothyroid group, the range was similar in patients with both primary and secondary hypothyroidism. There was no overlap between the three thyroidal states. Elevated T(3) levels were seen in 40 cases that appeared clinically hyperthyroid but had normal serum thyroxine (T(3)) determinations, a syndrome we have called T(3) toxicosis. Values obtained with radioimmunoassay agreed closely with those we had previously found by gas-liquid chromatography which were 68 +/-2 ng/100 ml in hypothyroidism, 137 +/-23 ng/100 ml in normal subjects, and 510 +/-131 ng/100 ml in untreated toxic diffuse goiter. Since T(3) is very potent and its level varies in different clinical states, accurate T(3) measurements are required to assess a patient's thyroid status properly. The radioimmunoassay for T(3) appears to be sufficiently sensitive, precise, and simple to permit its routine clinical application for this purpose.

Radioimmunoassay for measurement of triiodothyronine in human serum

A convenient, specific, precise, and reproducible radioimmunoassay system for measurement of triiodothyronine (T(3)) in human serum has been developed. The procedure compares the ability of standards and unknowns to compete with radioactive T(3) for binding sites on a T(3)-binding antiserum produced in rabbits by immunization with human thyroglobulin. The assay is set up in the presence of 250 ng thyroxine (T(4)) in all tubes, to mobilize T(3) from its binding with the thyronine-binding globulin (TBG), and athyreotic sheep serum in standards to correct for the TBG in the unknowns. The method regularly detected 0.4 ng T(3), which would correspond to a T(3) concentration of 100 ng/100 ml when 400 mul of serum is analyzed. The mean recovery of unlabeled T(3) added to normal serum pools was 106%. Serial dilution of hyperthyroid sera containing high concentrations of T(3) with athyreotic sheep serum yielded expected values. The serum T(3) concentration in 80% of 31 euthyroid normal subjects was less than 100 ng/100 ml (range < 100-170 ng/100 ml); it was greater than 170 ng/100 ml in 89% of 27 sera of hyperthyroid patients with untreated Graves' disease (range < 100-1300, mean 519 in 25 sera with detectable T(3)). The concentration of serum T(3) fell, frequently to undetectable levels, during treatment of hyperthyroid patients with antithyroid drugs. The serum T(3) concentration in four hypothyroid patients was less than 100 ng/100 ml.