Effect of oleate, diphenylhydantoin and heparin on the binding of 125 I-thyroxine to purified thyroxine-binding globulin

Interaction of thyroid hormone and heparin in postischemic myocardial recovery

BACKGROUND

Triiodothyronine (T3) administration can improve postischemic myocardial recovery. Heparin can interfere with cellular binding of T3. Introduction of heparin into an isolated heart model may interfere with this effect.

METHODS

Four groups of 8 rat hearts were placed on a modified Langendorff apparatus. All groups underwent 15 minutes of perfusion with modified Krebs-Henseleit solution (KH), followed by 20 minutes of normothermic global ischemia and 30 minutes of reperfusion. Group I underwent reperfusion with KH. Group II underwent reperfusion with KH and 1 x 10(-6) mol/L of T3. In group III, hearts underwent preischemic perfusion with heparinized KH (1,000 U/L) and reperfusion with KH containing 1 x 10(-6) mol/L of T3 and 1,000 U/L of heparin. In group IV, rats were given heparin at 2,000 IU/kg 30 minutes before sacrifice, and isolated hearts were reperfused with KH and 1 x 10(-6) mol/L of T3. A latex balloon in the left ventricle monitored hemodynamic variables.

RESULTS

Left ventricular developed pressure throughout postischemic reperfusion was greater in all the groups receiving T3 when compared with group I. Group II showed significantly greater recovery than either group III (p < 0.05) or group IV (p < 0.05).

CONCLUSIONS

Addition of T3 to the reperfusate enhances postischemic myocardial recovery in the isolated heart model, whereas addition of heparin reduces this effect.

Cardiopulmonary bypass and thyroid hormone metabolism

Early investigations involving patient response to thyrotropin-releasing hormone during cardiac operations prompted researchers to consider that the cardiopulmonary bypass (CPB) procedure may affect this response. Results from several studies indicate that total T3 (active thyroid hormone) concentrations are significantly reduced during and after CPB (ie, the euthyroid sick syndrome). Inhibition of the monodeiodinase pathway during CPB, and subsequent inhibition of peripheral thyroxine to T3 conversion may partly explain these findings. These data prompted the investigation of intravenous T3 administration to patients undergoing CPB. Clinical trials to date have shown that intravenous T3 administered during or after CPB improves cardiac output and contractility. This article reviews the studies of thyroid hormone before, during, and after CPB operations.

Mechanism of the serum thyroid hormone lowering effect of perfluoro-n-decanoic acid (PFDA) in rats

The mechanism and consequences of the serum thyroid hormone lowering effect of perfluorodecanoic acid (PFDA) were examined. Thyroid and pituitary gland functions in PFDA-treated rats were assessed by measuring radioiodine uptake from the circulation and the ability of the thyroid gland to secrete thyroxine (T4) and triiodothyronine (T3) in response to thyrotropin-releasing hormone (TRH) stimulation. Serum levels of reverse triiodothyronine (rT3) were measured to test for possible conversion of T4 to a biologically inactive product and the displacement of radiolabeled T4 from rat albumin in vitro by PFDA was examined. Finally, changes in activity of the thyroid hormone-sensitive liver enzymes glycerophosphate dehydrogenase (GPD) and malic enzyme (ME) in response to PFDA were analyzed. Functional activities of the thyroid and/or pituitary glands appear to be somewhat depressed by PFDA treatment. There was no increased conversion of T4 to rT3. PFDA displaced radiolabeled T4 from rat albumin with an affinity similar to thyroxine. The activities of both GPD and ME were significantly increased in livers from PFDA-treated rats. These results suggest that decreased serum levels of thyroid hormones may be due to (1) reduced responsiveness of the thyroid and/or pituitary glands to hormonal stimulation and (2) a displacement of circulating hormones from plasma protein binding sites by PFDA. Increased activity of the liver enzymes GPD and ME does not reflect the reduction in circulating thyroid hormones and indicates that PFDA-treated rats are apparently not functionally hypothyroid at the tissue level.

[Free fatty acids in the serum in critical diseases: do they play a role in the protein binding of thyroid hormones?]

As a contribution to the question if the elevated concentrations of free fatty acids in sera of critically ill patients described in literature play a role in the decrease of thyroid hormone levels in these patients, serum levels of the important free fatty acids were measured in 31 patients of our intensive care unit in the course of their disease using gas chromatography. After admission to the ward, only palmitoleic acid was significantly increased compared with 174 control persons, arachidonic acid was not different from the controls, palmitic, stearic, linoleic and linolenic acid were significantly decreased. In the course of the disease, no relevant changes were observed. The 21 patients not surviving their disease showed significantly lowered levels of palmitic, stearic and linoleic acid before death compared with the surviving patients at the end of the observation period. The hypothetical role of single free fatty acids as inhibitors of the binding of thyroid hormones to their transport proteins must be questioned because of the results.

Effect of heparin on serum free thyroxine linked to post-heparin lipolytic activity

A possible association between free T4 (fT4) changes, occurring after heparin administration, and the post-heparin lipolytic activity, i.e., the release of lipases and the lipolysis of triglycerides to non-esterified fatty acids (NEFAs) was studied. In 19 patients heparin increased mean fT4 values obtained by equilibrium dialysis from 23.4 to 38.6 pmol/l (P less than 0.01) and mean fT4 values measured by GammaCoat two-step RIA from 17.0 to 24.1 pmol/l (P less than 0.01), the results of the two methods agreeing well (rs = 0.88). Concurrently, NEFAs increased from a mean of 0.55 to 2.20 mmol/l (P less than 0.01) and fT4 increases were significantly correlated with post-heparin NEFAs (rs = 0.69, P less than 0.001). In vitro addition of palmitic acid to sera increased fT4 concentrations to values similar to those observed in vivo. Post-heparin fT4 increases were also strongly correlated to the concentrations of triglycerides (rs = 0.63, P less than 0.01). Hypertriglyceridaemia was associated with pronounced fT4 increases after heparin administration, and normolipidaemia with moderate or no changes. These results suggest that the effect of heparin on fT4 is linked to the activation of lipases by heparin and is mediated by NEFAs, which, at high concentrations, compete with thyroxine for binding proteins.

Mechanisms of interference of nonesterified fatty acids in radioimmunoassays of steroids

Addition of nonesterified fatty acids caused an apparent increase in unbound steroid present in supernatants in radioimmunoassays for steroids utilizing dextran-coated charcoal. Nonesterified fatty acid interference occurred at the initial binding interaction between steroid and its antiserum, and also at the step separating bound from unbound steroid. It was determined that nonesterified fatty acids, which had been added to radioimmunoassays, formed micelles and trapped steroids. The extent of the entrapment was inversely related to the number of polar groups in the steroid molecule, that is, hydrophobic steroids more easily interacted with the nonesterified fatty acid micelles. However, if the charcoal concentration were increased, the nonesterified fatty acid effect was eliminated for assays of polar steroids and greatly reduced for non-polar steroid assays.

The effect of intravenous heparin infusions on the thyroid stimulating hormone response to thyrotrophin releasing hormone

1 The TSH response to TRH is suppressed 24 h after the commencement of i.v. heparin infusion in euthyroid subjects, as compared to repeat testing, both after 7 days of continuous i.v. heparin and after 7 days without heparin but while on warfarin. 2 This suggests that the rise in serum free thyroid hormones, which heparin produces, has a 'metabolic effect'. Possible clinical effects of this action must be considered. 3 Heparin has no effect on the radio-immunoassay of TSH.

Minimal effects of free fatty acids on the competitive protein-binding assay of serum thyroxine on reusable Sephadex columns

In rats; following oral administration of cream and intravenous heparin, plasma free fatty acids and triiodothyronine Sephadex uptake were elevated. Simultaneously apparent serum thyroxine, measured by the Tetrasorb-E kit, was increased, but not when measured using reusable Sephadex columns. Addition of oleate to a human serum albumin solution or to human serum produced similar effects; appreciable rises in apparent thyroxine were noted in the kit assay but only minor changes occurred when Sephadex columns were used. In the latter procedure almost all [14C]oleic acid added to serum was removed in the first barbital buffer wash, before application of the thyroxine-binding globulin solution. In the serum thyroxine assay on reusable Sephadex columns, there is minimal interference by free fatty acids because they are removed by the first barbital buffer wash.

Effect of fatty acids on thyroid function tests in vitro and in vivo

Addition of long-chain fatty acids to serum increased thyroxine (T4), measured by a competitive protein binding assay, and triiodothyronine (T3) uptake by Sephadex or resin (T3U tests). This is compatible with the assumption that fatty acids compete with thyroxine for binding sites on T4-binding proteins. When equimolar concentrations of various saturated and unsaturated fatty acids were added to serum it was observed that the effectiveness in raising tests based on protein binding of thyroid hormones incrreased serum T3 determined by radioimmunoassay (RIA). T4(RIA) was not significantly influenced by either saturated or unsaturated fatty acids. Serum T4(CPB) rose during storage at 22degreesC and 37degreesC but was stable at 4degreesC and --20degreesC for periods up to two weeks. The proportional increase in T4(CPB) and free fatty acids (FFA) indicated that this phenomenon was due, at least partly, to the interference from FFA formed during storage of the serum. There was also a small, significant increase in T3U, T3(RIA) and CT4I (a free thyroxine estimate) after storage of serum at room temperature or higher for one to two weeks. Serum T4(RIA) did not alter during two weeks of storage. In five subjects with raised serum FFA after eating a fat meal followed by a heparin injection an increase in T4(CPB), T3U, T3(RIA) and CT4I that was proportional to the increase in FFA was observed. This effect on the thyroid tests was small until the increase in FFA concentration exceeded 2 mmol/l. T4(RIA) did not respond to the increase in FFA. In ten patients with raised levels of FFA due to uncontrolled diabetes T4(CPB), T4(RIA) and T3(RIA) decreased while T3U increased. These unexpected alterations were probably related to the severe, chronic illness in these patients. Increased FFA in vivo seem to be of little importance for the interpretation of thyroid tests in clinical practice.