Lack of significant binding of L-triiodothyronine by thyroxine-binding globulin in vivo as demonstrated by acute disappearance of 131-I-labeled triiodothyronine

Reciprocal effects of an acute load of thyroxine or triiodothyronine on their peripheral metabolism and deiodination in the cold-acclimated rat

Wistar rats acclimated to cold received iv 25 micrograms T 4/100 g bw, 21 micrograms T 3/100 g bw or 2.5 micrograms T 3/100 g bw, to observe the changes induced in the peripheral metabolism of 125I-T4 and 125I-T3 relative to cold-adapted untreated animals. Each animal was injected with a tracer dose of either labelled hormone in addition to the T4 or T3 load, and placed in metabolic cages for 24 hour collection of urine and feces. Sequential heparinized blood samples were obtained by cardiac puncture. A T4 load decreased the deiodination of 125I-T4 and 125I-T3 (p less than 0.01) as revelated by urinary 125I. A T3 load, in the two dosages employed, decreased the deiodination of 125I-T3 (p less than 0.01) but had no effect on deiodination of 125I-T4. Similarly, a T4 load increased the fecal excretion of both radioactive iodothyronines (p less than 0.01) whereas a T3 load failed to alter the excretion of 125I-T4. In cold-adapted animals plasma TSH was elevated (p less than 0.05) and plasma T4 was low (p less than 0.001) as compared to rats housed at 22 C. It is concluded that the relative contribution of T4 and T3 to the metabolic state in the rat is not significantly altered by cold exposure.

Influx of thyroid hormones into rat liver in vivo. Differential availability of thyroxine and triiodothyronine bound by plasma proteins

The transport of [(125)I]thyroxine (T(4)) and [(125)I]triiodothyronine (T(3)) into liver was investigated with a tissue sampling-portal vein injection technique in the anesthetized rat. The method allows the investigation of the effects of plasma proteins in human serum on the unidirectional influx of T(4) or T(3) into liver cells. The percent extraction of unidirectional clearance of T(3) and T(4) was 77+/-2% and 43+/-2%, respectively, after portal injection of a bolus of Ringer's solution. Cell membrane transport of T(4) or T(3) was nonsaturable because 50-muM concentrations of unlabeled hormone had no effect on transport. The addition of bovine albumin in concentrations of 1, 5, or 10 g/100 ml bound >98% of T(4) or T(3) in vitro, but had no significant effect on T(3) or T(4) transport in vivo. Conversely, 10% rabbit antisera specific for T(3) or T(4), completely abolished the intracellular distribution of thyroid hormone into liver. In the presence of rat serum, which contains albumin and thyroid hormone binding pre-albumin (TBPA), 18 and 81% of total plasma T(4) and T(3), respectively, were available for transport in vivo. The fraction of hormone available for transport in the presence of normal human serum, which contains albumin, TBPA, and thyroid hormone binding globulin (TBG) was 11% for T(4) and 72% for T(3). The fraction of hormone transported into liver after injection of serum obtained from pregnant or birth control pilltreated volunteers was 4% for T(4) (but this was not significantly different from zero) and 54% for T(3). THESE DATA SUGGEST: (a) The mechanism by which T(4) and T(3) traverse the liver cell membrane is probably free diffusion. (b) Albumin-bound T(4) or T(3) is freely cleared by liver, approximately 50% of TBG-bound T(3) is transported, but little, if any, of TBPA-bound T(4) or TBG-bound T(4) is cleared by liver cells. (c) Although the albumin-bound fraction of T(4) greatly exceeds the free (dialyzable) moiety, the two fractions are both inversely related to the existing TBA or TBG level; therefore, in vitro measurements of free T(4) would be expected to accurately reflect what is available for transport in vivo. Conversely, TBG-bound T(3) is readily transported in vivo; therefore, it is proposed that in vitro measurements of free T(3) do not reliably predict the fraction of T(3) available for transport into liver in vivo.

Changes in the T4/T3 molar ratio following thyrotropin releasing hormone injection in cattle

The injection of thyrotropin releasing hormone into cattle resulted in a rapid decrease in the T4/T3 molar ratio. 2 breeds of cattle, Shorthorn and Africander Cross were studied. The decrease in the T4/T3 molar ratio was significantly greater in the Shorthorn breed. It is concluded that acute stimulation of the thyroid gland with TRH results in enhanced release of both T3 and T4 and that T3 is discharged more rapidly than T4.

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.

The distribution kinetics of triiodothyronine: studies of euthyroid subjects with decreased plasma thyroxine-binding globulin and patients with Graves' disease

The kinetics of distribution of 3,3',5-triiodo-L-thyronine (T(3)) have been studied employing both a single-injection and a continuous infusion of T(3-) (131)I. External monitoring of radioactivity in the liver during the infusion permitted estimation of the hepatic distribution volume (V(H)) and the one-way hepatic clearance (C(H)) of the hormone. Among 10 euthyroid control subjects, V(H) averaged 2.07 liters +/-0.50 (SD), and the mean value for C(H), 231 ml of plasma per min (+/-64). In three euthyroid men whose plasma showed decreased binding capacity by thyroxine-binding globulin (TBG) abnormally high V(H) and C(H) values were found, the increase in C(H) being proportional to the decrease in binding activity by plasma proteins. Among all 13 subjects, there was a high correlation (+ 0.86) between C(H) and the proportion of free hormone in plasma, measured in vitro. In four patients with hyperthyroid Graves' disease V(H) ranged from 3.2 to 4.2 liters and C(H) was elevated in every case, averaging 989 ml/min. The increase in C(H) in this group was out of proportion to the elevation of free hormone fraction in plasma. Seven patients who were either euthyroid or hypothyroid after treatment of Graves' disease showed a slight but significant increase in C(H) compared with the euthyroid controls without Graves' disease. The percentage of free hormone in the plasma of the treated group was normal or low and therefore could not explain the persistent elevation in unidirectional hepatic clearance observed. The rate of accumulation of labeled T(3) in the tissues of the thigh during the interval from 10 to 60 min of the sustaining infusion of tracer was slow compared to the rate of equilibration in the liver and did not differ significantly among the various groups studied. These latter findings suggest that in slowly equilibrating tissues such as the thigh the kinetics of T(3) distribution are relatively insensitive to alterations in hormone-binding activity by plasma proteins.

The effects of an acute load of thyroxine on the transport and peripheral metabolism of triiodothyronine in man

In order to examine the question of whether thyroxine-binding globulin (TBG) influences significantly the peripheral metabolism of 3,3',5-triiodo-L-thyronine (T(3)) in vivo, paired studies of the effects of a large intravenous load of L-thyroxine (T(4)) on the kinetics of (131)I-labeled T(3) metabolism were carried out in five normal subjects. After the T(4) load, both the early distributive loss of labeled T(3) from serum and the volume of T(3) distribution, observed after distribution equilibrium had been attained, were greatly increased. These alterations were consistent with those to be expected from displacement of T(3) from its extracellular binding sites. After the T(4) load, however, the fractional rate of T(3) turnover was decreased. This finding is ascribed either to competition between T(3) and T(4) for common intracellular pathways of degradation or excretion or to displacement of T(3) from sites of more rapid to sites of less rapid metabolism. These effects of alterations in the binding activity of TBG on the peripheral metabolism of T(3), together with those previously reported by others, are consistent with the interpretation that T(3) is significantly bound by TBG in vivo. However, it is suggested that the effects of alterations in the T(3)-TBG binding interaction on the metabolism of T(3) are obscured by alterations in the extracellular-cellular partitioning of T(4) that would result from concurrent alterations in T(4)-binding by TBG.