J. R. Stockigt

LIMITATIONS OF A NEW FREE THYROXINE ASSAY (AMERLEX® FREE T4)

We have assessed a new method of free T4 measurement (Amerlex®) which uses a novel unidentified T4‐labelled analogue, said to be unreactive with T4 binding proteins in serum, together with an antibody that binds both analogue and T4. Free T4 is assessed by competition with analogue for antibody binding‐sites. The test method has been compared with free T4 measured by equilibrium dialysis and with a technique using an immobilized T4 antibody. All methods gave the expected free T4 levels in normal, hyperthyroid and hypothyroid subjects and normal free T4 levels with high or low levels of T4 binding globulin. However, in autosomal dominant familial euthyroid T4‐excess, where T4 is abnormally bound to albumin, the test method gave apparent high free T4 levels suggestive of hyperthyroidism. In a selected group of severely‐ill euthyroid patients the new method gave apparent low free T4 levels. In view of these discrepancies, binding of labelled analogue was evaluated by dextran‐charcoal separation at 4°C. Familial euthyroid T4‐excess sera showed greater analogue binding and samples with low prealbumin concentration showed less binding than did normal sera. Despite its validity with variations in TBG, it appears that Amerlex® Free T4 is influenced by lower‐affinity, high‐capacity T4 binding sites in serum, so that apparent free T4 concentration may vary with changes in the concentration of such sites.

Post-partum hypercalcemia in hereditary hyperphosphatasia (juvenile Paget’s disease)

Hereditary hyperphosphatasia is a rare bone disorder characterized by increased bone turnover, elevated alkaline phosphatase (ALP) and bone deformity. We describe a patient with a mild form of hereditary hyperphosphatasia who was initially hypercalcemic in childhood with remission after puberty. Symptomatic hypercalcemia recurred during lactation after each of two pregnancies, associated with increased bone turnover (rise in ALP, osteocalcin, and urine hydroxyproline excretion) which appeared to be independent of changes in major calcium-regulating hormones. The mechanism for the development of post-partum hypercalcemia remains unclear but may relate to the relative estrogen deficiency of lactation. We postulate that acute estrogen withdrawal may result in hypercalcemia in the presence of markedly increased bone turnover.

An adrenal adenoma causing virilization of mother and infant.

A right adrenal androgen‐producing adenoma was identified by CT scanning in a 33‐year‐old woman after delivery of an otherwise‐normal, virilized female infant. Despite clinical evidence of mild chronic androgen excess, maternal reproductive function was normal. Post‐partum studies showed 2–5‐fold excess in maternal plasma testosterone, androstenedione and dehydroepiandrosterone‐sulphate; urinary androgen metabolites were increased 3–10‐fold. Androgen production from the tumour increased acutely in response to hCG administration; after removal of the tumour, androgen levels were normal and showed negligible responses to hCG. These findings lead us to speculate that endogenous chorionic gonadotrophin may have led to augmented androgen production from the adrenal tumour during pregnancy, causing fetal virilization as previously described in patients with ovarian tumours. This study demonstrates that CT scanning can be valuable in distinguishing between adrenal and ovarian androgen‐producing tumours, a distinction which is often unreliable when based on physiological manipulations of hormone secretion.

Furosemide, fenclofenac, diclofenac, mefenamic acid and meclofenamic acid inhibit specific T3 binding in isolated rat hepatic nuclei

Previous studies with Phenytoin (DPH) show that this inhibitor of thyroid hormone binding to plasma proteins also interacts with specific nuclear T3 binding sites. In order to further define the nuclear effects of drugs that inhibit plasma protein binding of thyroid hormones, we assessed furosemide and a number of non-steroidal antiinflammatory drugs using isolated rat liver nuclei. The effects were compared with those of DPH, ipodate and amiodarone. The T3 binding site in isolated nuclei (Ka 1.2×109M−1) showed relative affinity triac ≈ T3>T4. Drugs were studied over the concentration range 10−3-10−7M, approximating the known therapeutic total plasma concentrations, in competition with 125|-T3 0.1 nM, expressing inhibition as the percent decrement from maximum specific binding of 125|-T3 in drug vehicle (assay buffer or ethanol 1–10%). Specific T3 binding was inhibited by furosemide to 78.8 ± 3.5% at 2 mM, by fenclofenac to 37.6 ± 2.8% at 1 mM, by meclofenamic acid to 70.2 ± 2.4% at 0.1 mM, by mefenamic acid to 60.6 ± 4.6% at 0.05 mM (each p< 0.02) and by diclofenac to 87.4 ± 5.6% at 0.2 mM (p < 0.05). In comparison, DPH inhibited T3 binding to only 88.1 ± 0.6% at 0.3 mM, as did calcium ipodate (68 ± 3.5% at 1 mM, p<0.02). Amiodarone (0.3 mM), sodium salicylate (1 mM) and phenylbutazone (0.1 mM) were inactive. In order to achieve a level of nuclear receptor occupancy that approaches in vivo occupancy, the concentration 125|-T3 was increased over the range 0.1–0.5 nM. At constant drug concentrations, the decrease in the concentration of bound 125|-T3 induced by furosemide, fenclofenac, diclofenac, mefenamic acid, and meclofenamic acid and ipodate was accentuated by increased receptor occupancy, as previously demonstrated with DPH. These studies demonstrate that a wide range of bicyclic drugs may interact with specific nuclear T3 binding sites, over a range of total concentrations similar to those previously reported as inhibitory for DPH and ipodate. The molecular basis for the relationship between inhibition of specific nuclear receptor and plasma protein binding remains unknown, and the potential for these drugs to alter expression of thyroid hormone action is not yet defined. Effects on TSH secretion, previously attributed to changes in circulating free hormone concentration, could also result from interaction with nuclear receptors. It will be necessary to consider the relationship between free intranuclear drug and T3 concentrations to further define these potentially-important pharmacologic interactions.

IOPANOIC ACID IS OF MINIMAL BENEFIT IN THE TREATMENT OF SEVERE HYPERTHYROIDISM: CONCLUSIONS FROM A CASE STUDY

Iopanoic acid (1 g/d) was used together with propylthiouracil (1200 mg/d) in the treatment of a patient with very severe hyperthyroidism and associated cardiac failure. Although serum total T3 decreased by 75% within 48 h and reached normal after 72 h, free T3 levels did not fall to normal. Total and free T4 remained markedly elevated and features of hyperthyroidism persisted. Estimations of theoretical in vivo occupancy of nuclear thyroid hormone receptors, based on serum free T4 and free T3, suggest that the marked decrease in total T3 would not result in a corresponding decrease in thyroid hormone action. Hence, estimates of potential benefit from oral cholecystographic contrast agents, based only on measurements of total T3, may be unduly optimistic. When temporary agranulocytosis developed in this patient, the prior use of iopanoic acid, by markedly reducing thyroidal iodine uptake, restricted the therapeutic options. Caution should, therefore, be exercised in the use of iodine‐containing contrast media as adjunctive antithyroid agents.

FAMILIAL DYSALBUMINAEMIC HYPERTHYROXINAEMIA: STUDIES OF ALBUMIN BINDING AND IMPLICATIONS FOR HORMONE ACTION

The abnormal intermediate‐affinity T4 binding to albumin which is characteristic of familial dysalbuminaemic hyperthyroxinaemia (FDH) is dependent on buffer, temperature, and ionic composition. Scatchard analysis of T4‐binding to isolated albumin preparations from FDH subjects showed that half the circulating albumin showed the higher‐affinity T4 binding site, assuming one site per molecule. Using dextran‐charcoal separation at 40C the T4 affinity (Ka) of purified albumin from FDH subjects was 7.5 nmol/l in phosphate and 17 nmol/l in Tris‐CI‐ buffer. T4 binding to FDH albumin was inhibited by a range of substances in the order: 8‐anilino‐l‐naphthalene sulphonic acid > merthiolate>propylthiouracil> methyl‐thiouracil > carbimazole > salicylate > barbitone. Binding of T4 was competitively inhibited by low concentrations of dithiothreitol (DTT). The effect of DTT 0.1–0.5 mmol/l was reversed by removal of DTT by dialysis. Competition with a range of iodothyronines indicated that the 3′, 5′‐iodine atoms are most important for binding to this site. Serum binding of salicylate. frusemide. fenclofenac and barbituric acid, and a range of steroid hormones was similar in FDH and normal sera. Serum levels of sex hormone binding globulin (SHBG), were not significantly different from sex‐matched controls. Nuclear [125I]‐T3 binding sites in circulating lymphocytes from two FDH subjects showed affinities (Kd) of 59 and 79 pmol/l (normal 67±7 pmol/l, n= 6. These findings suggest that the highly specific binding anomaly of FDH is due to a disulphide‐dependent structural change in albumin. The normal T3 affinity of lymphocytic receptors and normal SHBG levels are consistent with a normal relationship between free hormone and tissue response in FDH.

Familial dysalbuminaemic hyperthyroxinaemia and inherited partial TBG deficiency: first report

BACKGROUND Abnormalities of the serum thyroid hormone binding proteins are not uncommon but, when properly asessed, they do not present diagnostic difficulties. In contrast, the presence of two inherited defects of thyroid hormone transport, of the type presented in the family described here, may cause a major problem in diagnosis and has not been described previously.

Transient Anterior Electrocardiographic Changes Simulating Acute Anterior Myocardial Infarction in Diabetic Ketoacidosis

In two patients with severe diabetic ketoacidosis, electrocardiography showed transient anterior changes suggestive of acute transmural infarction without subsequent evidence of myocardial necrosis. While the mechanism of these and other temporary electrocardiographic changes in diabetic ketoacidosis remains unclear, appreciation of their transient nature is essential if misdiagnosis of myocardial infarction and possible inappropriate delay in intravenous fluid administration are to be avoided. When electrocardiographic abnormalities are present early in diabetic ketoacidosis, the full 12-lead electrocardiogram should be repeated after adequate resuscitation.

Mineralocorticoid activity of 21-deoxyaldosterone derivatives: structure-function studies.

Abstract The synthesis of 21-deoxyaldosterone was undertaken to determine whether it has a possible role as a mineralocorticoid antagonist in human metabolism. It was found that the synthetic 11,18-hemiacetal form of 21-deoxyaldosterone could be converted to a 20-methyl ether when exposed to silica gel and methanol during high performance liquid chromatography. The preparation of a 11,18-acetal-18,20 hemiketal isomer and other derivatives of 21-deoxyaldosterone are described and proton n.m.r. data presented. The 20-methyl ether of 21-deoxyaldosterone has one third the affinity of aldosterone for rat kidney mineralocorticoid receptors; on bioassay, however, it shows only ~ 1% the agonist activity of aldosterone, and no antagonist activity. 21-Deoxyaldosterone 18-acetate, the 18-methyl ether and the free 20-hemiketal isomer, in contrast, have low affinity for mineralocorticoid receptors (~2% that of aldosterone) 17-iso-21-deoxyaldosterone has negligible affinity. Before possible (patho)-physiological roles can be assigned to any naturally occurring 21-deoxy metabolite of aldosterone, therefore, it would appear necessary to establish both its isomeric state and in vivo stability

Evaluation of Oleic Acid, Arachidonic Acid, and Various Drugs as Competitors for Serum Binding of Thyroxine

Numerous drugs can displace thyroid hormones from plasma protein binding (1,2), and it has been suggested that substances of endogenous origin can also inhibit binding, particularly in severe non-thyroidal illness (35). The potency of a competitor is influenced by at least three factors: total circulating concentration, free fraction in undiluted serum, and relative affinity for T4 binding sites. From these three factors, a prediction of inhibitory potency can be made. This prediction can be tested by measuring the effect of circulating concentrations of potential inhibitors on 125I T4 binding in undiluted serum at 37°C. In this study, we examine these predictions for furosemide, various nonsteroidal anti-inflammatory drugs, and free fatty acids (FFA), which have been assessed because they appear to be promising candidates as thyroid hormone-binding inhibitors (THBI) (3). Recent studies suggest that oleic acid may contribute more importantly to THBI of non-thyroidal illness than other FFA (4). In these studies (3–5), binding was assessed using diluted serum in a competitive ligand binding assay. We have examined the effect of serum dilution on the apparent inhibitory potency of added FFA in order to evaluate effects seen in such assays.