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

Determination of iodothyronine absorption and conversion of L-thyroxine (T 4 ) to L-triiodothyronine (T 3 ) using turnover rate techniques

The absorption of L-thyroxine (T(4)) and L-triiodothyronine (T(3)) and the fractional rate of conversion of T(4) to T(3) were determined from the turnover rates of T(4) and T(3) in seven patients without endogenous thyroid function during separate treatment periods with these iodothyronines. Serum T(3) concentration was measured by a radioimmunoassay procedure in which the iodothyronines are separated from the plasma proteins before incubation with anti-T(3) antibody. Metabolic clearance rates were calculated by an integral (noncompartmental) approach since the use of single compartment kinetics led to a 40% overestimation of the metabolic clearance rate of T(3). Based on the amount of hormone ingested and the observed hormonal turnover rates, the absorption of T(4) and T(3) (iodothyronine turnover/iodothyronine ingested) in man could be estimated. Absorption of T(3) was complete in three subjects but decreased to 43% in a fourth who was suffering from mild congestive heart failure. Mean T(4) absorption was 48.0+/-2.6% (SEM) for seven subjects. The mean fractional rate of T(4) to T(3) conversion determined during T(4) replacement therapy (T(3) turnover/T(4) turnover) was 42.6% (range 30.7-50.8%). Thus, approximately one-half of the T(4) which was deiodinated was converted to T(3) suggesting that monodeiodination is an obligatory step in the peripheral metabolism of T(4). Calculations based on these results together with other available data suggest that under normal physiologic circumstances the major portion of the T(3) pool is derived from monodeiodination of T(4).

Radioimmunoassay of triiodothyronine in unextracted human serum

Serum triiodothyronine (T-3) concentrations have been estimated by radioimmunoassay using unextracted serum. The serum T-3 concentrations have been shown to be similar in two separate European populations (0.76-1.67 ng/ml). Raised T-3 values have been observed in all subjects with hyperthyroidism. Low values are seen in hypothyroidism although there is some overlap with the normal range. There is a good correlation between serum T-3 and serum thyroxine (T-4) concentrations, and estimation of T-3 seems likely to prove a practical and reliable test of thyroid function.

Modulation of pituitary responsiveness to thyrotropin-releasing hormone by triiodothyronine

The relative roles of triiodothyronine (T(3)) and thyroxine (T(4)) in modulating pituitary responsiveness to thyrotropin-releasing hormone (TRH) have been assessed. (a) 10 hyperthyroid patients with elevated serum T(2) and T(4) levels showed no pituitary response to TRH. After 2 wk of propylthiouracil therapy T(4) levels had fallen to normal in only five patients while T(2) levels were normal in all. Pituitary responsiveness to TRH returned in all patients with normal or high T(4) concentrations. (b) Patients with isolated elevations of serum T(3) (T(3) toxicosis) failed to respond to TRH. TRH responsiveness was restored when T(3) levels fell to normal after propylthiouracil therapy. (c) When pituitary responsiveness to TRH was tested 60 min after a single oral dose of 50 mug of T(3), which increased serum T(3) levels to slightly above the normal range, no rise in thyrotropin (TSH) was seen in six subjects. These findings indicate that T(3) elevations alone can rapidly inhibit pituitary responsiveness to TRH.

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.

Thyrotropin-releasing hormone: evidence for thyroid response to intravenous injection in man

Administration of thyrotropin-releasing hormone to normal subjects causes a prompt rise in plasma thyrotropin concentration, followed by a significant increase in circulating plasma triiodothyronine. These observations may prove to be of value in simultaneously assessing the ability of the pituitary and thyroid glands to respond to their trophic hormones.