High concentrations of furosemide inhibit serum binding of thyroxine

Structural aspects of thyroid hormone binding to proteins and competitive interactions with natural and synthetic compounds

Thyroid hormones and their metabolites constitute a vast class of related iodothyronine compounds that contribute to the regulation of metabolic activity and cell differentiation. They are in turn transported, transformed and recognized as signaling molecules through binding to a variety of proteins from a wide range of evolutionary unrelated protein families, which renders these proteins and their iodothyronine binding sites an example for extensive convergent evolution. In this review, we will briefly summarize what is known about iodothyronine binding sites in proteins, the modes of protein/iodothyronine interaction, and the ligand conformations. We will then discuss physiological and synthetic compounds, including popular drugs and food components, that can interfere with iodothyronine binding and recognition by these proteins. The discussion also includes compounds persisting in the environment and acting as endocrine disrupting chemicals.

Drug-induced endocrine disorders in the intensive care unit

The neuroendocrine response to critical illness is key to the maintenance of homeostasis. Many of the drugs administered routinely in the intensive care unit significantly impact the neuroendocrine system. These agents can disrupt the hypothalamic-pituitary-adrenal axis, cause thyroid abnormalities, and result in dysglycemia. Herein, we review major drug-induced endocrine disorders and highlight some of the controversies that remain in this area. We also discuss some of the more rare drug-induced syndromes that have been described in the intensive care unit. Drugs that may result in an intensive care unit admission secondary to an endocrine-related adverse event are also included. Unfortunately, very few studies have systematically addressed drug-induced endocrine disorders in the critically ill. Timely identification and appropriate management of drug-induced endocrine adverse events may potentially improve outcomes in the critically ill. However, more research is needed to fully understand the impact of medications on endocrine function in the intensive care unit.

Medications that distort in vitro tests of thyroid function, with particular reference to estimates of serum free thyroxine

The combination of serum thyroid-stimulating hormone (TSH) with measurement of circulating thyroid hormones greatly improves sensitivity and specificity of thyroid diagnosis, but these assays are not impeccable. Estimation of serum free T4 conveniently accommodates variations in the concentration of thyroxine-binding globulin (TBG), but no current technique reliably reflects the in vivo free T4 concentration in numerous other situations. The effect of circulating competitors that increase T4 and T3 in vivo, in particular, many medications, is under-estimated by current free hormone estimates that involve sample dilution. Non-esterified fatty acids generated during sample storage and incubation can spuriously increase the measured free T4 estimate, especially after in vivo treatment with heparin. These artefacts are unlikely to be overcome by current assay strategies. Total serum T4, corrected for alterations in TBG concentration, gives a more robust estimate of thyroxine concentration than current methods of free hormone estimation and should now be reintroduced as the 'gold standard'.

Nonthyroidal illness syndrome or euthyroid sick syndrome?

OBJECTIVE

To characterize the nonthyroidal illness syndrome (NTIS) and to discuss various underlying potential biochemical mechanisms for this condition.

METHODS

The pertinent medical literature was reviewed, and studies of thyroid function in systemic non-thyroidal illnesses were summarized.

RESULTS

Abnormalities of thyroid function in the NTIS have been classified into four major categories: (1) low triiodothyronine (T3) syndrome, (2) a combination of low T3 and low thyroxine (T4), (3) high T4 syndrome, and (4) other abnormalities. The NTIS has been noted in essentially all severe systemic illnesses and after caloric deprivation, major operations, and administration of some drugs. Some mechanisms that may contribute to low serum T3 in the NTIS are decreased type I 5 -monodeiodinase in tissues, decreased uptake of T4 by tissues, decreased serum binding, increased reverse T3, alterations in selenium status, cytokines, and a decrease in thyrotropin. Decreased thyrotropin may also contribute to low T4 levels in NTIS, as may decreased serum T4-binding proteins, abnormalities in T4-binding globulin, and circulating inhibitors of binding of T4 to serum proteins. Although T4 treatment of patients with NTIS has yielded little improvement, administration of T3 has produced some beneficial effects.

CONCLUSION

Further studies should be conducted to determine appropriate patient populations, dose-response ratios, and possible adverse effects of treatment of the NTIS with T3.

Comparison of colloid, thyroid follicular epithelium, and thyroid hormone concentrations in healthy and severely sick dogs

OBJECTIVES

To compare serum concentrations of total thyroxine (TT4), free thyroxine (fT4), and thyroid-stimulating hormone (TSH), as well as measures of thyroid follicular colloid and epithelium, between groups of healthy dogs and severely sick dogs.

DESIGN

Cross-sectional study.

ANIMALS

61 healthy dogs and 66 severely sick dogs.

PROCEDURE

Serum samples were obtained before euthanasia, and both thyroid lobes were removed immediately after euthanasia. Morphometric analyses were performed on each lobe, and serum TT4, fT4, and TSH concentrations were measured.

RESULTS

In the sick group, serum TT4 and fT4 concentrations were less than reference range values in 39 (59%) and 21 (32%) dogs, respectively; only 5 (8%) dogs had high TSH concentrations. Mean serum TT4 and fT4 concentrations were significantly lower in the sick group, compared with the healthy group. In the healthy group, a significant negative correlation was found between volume percentage of colloid and TT4 or fT4 concentrations, and a significant positive correlation was found between volume percentage of follicular epithelium and TT4 or fT4 concentrations. A significant negative correlation was observed between volume percentages of colloid and follicular epithelium in both groups.

CONCLUSIONS AND CLINICAL RELEVANCE

TT4 and fT4 concentrations are frequently less than reference range values in severely sick dogs. Therefore, thyroid status should not be evaluated during severe illness. The absence of any significant differences in mean volume percentages of follicular epithelium between healthy and severely sick dogs suggests that these 2 groups had similar potential for synthesizing and secreting thyroid hormones.

Thyroid function in neonates with severe respiratory failure on extracorporeal membrane oxygenation

The object was to study thyroid function in neonates with severe respiratory failure on extracorporeal membrane oxygenation (ECMO) and determine whether abnormal thyroid function correlates with prognosis. Total and free thyroxine (T4, FT4), total and free triiodothyronine (T3, FT3), reverse triiodothyronine (rT3), thyroid-stimulating hormone, and thyroxine binding globulin were measured in 14 newborn infants with severe respiratory failure (age 1-30 days) from samples collected before anesthesia for cannula placement, at 30, 60, and 360 min after initiation of ECMO, and on days 2, 4, 6, and 8. The patients were divided into survivors and non-survivors for statistical analyses. No differences were noted between survivors and non-survivors in the pre-ECMO mean serum concentrations of the thyroid function tests analyzed. In nine survivors, mean serum T4, FT4, T3, FT3, and rT3 all declined significantly within 30-60 min after initiation of ECMO, compared to baseline values. The values for all mean serum concentrations recovered completely and exceeded baseline between days 2 and 8. In five non-survivors, the decline of all mean serum values was not statistically significant and recovery to baseline was not achieved. The ratios of mean serum concentration of rT3/FT3 were significantly different between survivors and non-survivors across all times during the ECMO course (p < 0.0005). These findings indicate that abnormalities in thyroid function occur in neonates with severe respiratory failure on ECMO and that the rT3/FT3 ratio correlates with prognosis over the ECMO course. Survival was associated with a significant reduction of serum thyroid hormone concentrations followed by recovery. We speculate that, in neonates with respiratory failure on ECMO, adaptive mechanisms which enhance survival include the capacity to down-regulate the pituitary-thyroid axis.

Direct and Indirect Free Thyroxine Assay Methods: Theory and Practice

Background: For the diagnosis of thyroid disease, measurement of “free hormone” is generally accepted as an appropriate measure. However, valid assays measuring the free fraction of thyroxine (FT4) ideally must perform without bias, despite large variations in the concentrations and affinities of serum T4-binding proteins in the population. Several approaches have been taken to overcome such bias, and these have created considerable controversy in the field over the past decade.

Approach: This review, from both a historical and an analytical standpoint, charts the progress made over more than 30 years in improvements to the performance of assays in common use for the measurement of FT4 in serum or plasma. It reexamines the theory behind early approaches to such assays [for example, the free thyroxine index (FTI) method], that preceded more accurate, two-step immunoassays or one-step analog techniques. It evaluates the continuous refinements to the latter assays that by now have largely supplanted the FTI approach and where the deficiencies that so exercised clinical chemists in the past have been virtually eliminated in the leading assays.

Content: The basic Mass Action theory underpinning all such methods is discussed by assessing how far each particular approach obeys the criteria the theory imposes. In this, it is not the intention of the review to dissect individual commercial or academic assays, but rather to give guidance where appropriate as to how any assay said to measure FT4 can be conveniently evaluated by those intending to use it. Examples are given where inappropriate tests may wrongly imply assay invalidity by misinterpreting how FT4 assays work.

Summary: Detailed knowledge of the underlying theory is essential when devising tests for direct FT4 assays, to ensure that such tests do not overstep the practical limits of assay validity.

Free thyroid hormone measurement. A critical appraisal

The main purpose of free T4 and free T3 assays is to distinguish reliably between thyrotoxicosis, hypothyroidism, and the euthyroid state, an objective that cannot be attained with assays of total T4 and T3 because of hereditary and acquired variations in the concentrations of binding proteins. Effective correction for changes in the serum concentration of TBG can be achieved with numerous types of free hormone estimate, but other changes in binding are not well accommodated. Despite remarkable methodologic ingenuity, no current method reflects the free T4 concentration in undiluted serum under in vivo conditions. Equilibrium dialysis, widely considered the reference method for free T4 measurement, is also subject to error, either preanalytic, owing to generation of NEFA in the sample leading to an overestimate of free T4, or analytic with underestimation of the effect of competitors to increase free T4. Current approaches to free T4 measurement are vulnerable to several method-dependent artifacts: abnormal albumin binding of T4 or of the assay tracer, the inhibition of T4 binding to TBG by medications, and the effects of critical illness, especially in heparin-treated patients, pregnancy, and the abnormalities in sick premature infants. Because of systematic variation between methods (i.e., whether a technique is albumin dependent or prone to incubation or dilution artifacts), it is essential to consider methodologic details in evaluating free T4 estimates in these situations and whenever estimates of free T4 are clinically discordant. False-positive abnormalities are more frequent than false-negative results. When free T4 results are correlated with the serum TSH concentration with attention to the assumptions that define this relationship, the majority of false-positive results can be readily identified. If a free T4 anomaly remains unexplained on repeat sampling, it is appropriate to use an alternative free T4 method that depends on a different assay principle and to correlate the result with an authentic total T4 measurement.

Serum substances that interfere with thyroid hormone assays in patients with chronic renal failure

OBJECTIVE

Serum thyroid hormone concentrations in patients with chronic renal failure are usually low, despite normal serum TSH levels. We investigated the effect on thyroid hormone assays of serum dialysable organic acids that are elevated in uraemic patients.

PATIENTS

Serum samples from 42 patients with chronic renal failure who were receiving haemodialysis and 37 sex- and age-matched healthy subjects were examined.

DESIGN AND MEASUREMENTS

Serum thyroid hormone concentrations were measured with an analogue radioimmunoassay (RIA), a labelled antibody assay, and an equilibrium dialysis/RIA method. Serum concentrations of organic acids were determined with high performance liquid chromatography.

RESULTS

Serum thyroid hormone levels determined by an analogue RIA and a labelled antibody assay in uraemic patients increased, and serum concentrations of organic acids decreased following haemodialysis. A significant association existed between serum free T3 (FT3) levels determined by an analogue RIA and serum concentrations of indoxyl sulphate (IS) prior to dialysis. There was also a significant association between serum free T4 (FT4) levels determined by an analogue RIA and serum concentration of IS and hippuric acid (HA) prior to dialysis. There was a significant association between the changes of serum concentrations of indole acetic acid (IAA) and FT4 concentrations prior to and following haemodialysis when determined by an analogue RIA. Serum FT3 and FT4 levels significantly decreased after the addition of IS to serum from healthy subjects when determined by an analogue RIA but not by a labelled antibody assay. Serum FT4 levels, but not FT3 levels, decreased after addition of IAA when determined by an analogue RIA. Serum FT4 concentrations determined by an equilibrium dialysis/RIA were significantly higher than those determined by the other two methods. The addition of IS, IAA, and HA to serum samples from healthy subjects significantly increased FT4 concentrations when determined by an equilibrium dialysis/RIA method.

CONCLUSIONS

Increased serum levels of indoxyl sulphate, indole acetic acid and hippuric acid in sera of uraemic patients may interfere with thyroid hormone measurements when an analogue radioimmunoassay is used. In contrast, there was little Interference with a labelled antibody assay. Dialysable organic acids may also interfere with thyroid hormone assays determined by an equilibrium dialysis/radioimmunoassay method.

Sick Euthyroid Syndrome

Critical illness causes multiple alterations in thyroid hormone concentrations in patients who have no intrinsic thyroid disease. These effects are nonspecific, and they relate to the severity of illness. Because a wide variety of illnesses tend to result in the same changes in serum thyroid hormone levels, such alterations in thyroid hormone indexes has been termed the sick euthyroid syndrome. These changes are rarely isolated, and they are often associated with alterations in other endocrine systems. Similar changes in endocrine function has been shown experimentally by administration of cytokines from the interleukin and interferon families, as well as tumor necrosis factor-α. Thus, the sick euthyroid syndrome should not be viewed as an isolated pathological event, but as part of a coordinated systemic reaction to illness that involves both the immune and the endocrine systems. Recovery from the illness usually results in resolution of the alterations in thyroid hormone parameters. Supplemental thyroid hormone therapy in patients with the sick euthyroid syndrome is of no benefit and is not indicated.

Thyroid disease in older patients. Diagnosis and treatment

Thyroid gland dysfunction is a relatively common clinical problem in elderly people and is associated with significant morbidity if left untreated. The clinical features of thyroid disease may be subtle, easily overlooked or misdiagnosed. Therefore, a high index of suspicion is necessary. If potentially serious sequelae are to be avoided, the selection and interpretation of thyroid function tests must be appropriate. It is particularly important to consider both the effect of concurrent illness and the effect of certain drugs on thyroid function tests. With recent methodological advances, thyroid function tests are now more reliable, though in certain situations they still need to be interpreted with caution. Once the diagnosis is established, the management of both hypothyroidism and hyperthyroidism is relatively simple and effective, though there are special considerations relating to elderly patients.

Alterations in hepatocyte uptake and plasma binding of thyroxine in nonthyroidal illness and caloric deprivation

Low plasma T(3) in severe illness is widely thought to be due principally to inhibition of 5'-deiodinase activity, but other factors also contribute to this response. Abnormal plasma constituents, namely, 3-carboxy-4-methyl-5-propyl-2-furan propanoic acid (CMPF) and indoxyl sulfate in uremia, and elevated bilirubin and nonesterified fatty acids (NEFA) can impair T(4) transport into hepatocytes, thereby contributing to the lowering of plasma T(3). Assessment of possible endogenous or exogenous inhibitors of T(4) binding to plasma proteins is prone to dilution-dependent artifacts, which can lead to overestimation or underestimation of competitor potency, depending on experimental details. Because the potency of such competitors is a function of their free concentrations in undiluted serum, inhibitory activity may be enhanced by substances that impair their albumin binding. Oleic acid or CMPF can inhibit the effect of drugs such as furosemide or fenclofenac.

Critical evaluation of fully automated enzyme immunoassays for free thyroxine and thyrotropin

The technical and diagnostic performance of fully automated immunoassays for free thyroxine and thyrotropin using streptavidin/biotin technology (Enzymun-Test) were examined. The between-assay precision for free thyroxine was 10.4%, 5.4%, 2.5%, 2.3%, 1.1%, and 1.8% at 3.02, 6.27, 17.2, 21.9, 25.6, 42.7 pmol/l; and for thyrotropin was 14.2%, 4.7%, 2.9%, 2.8%, 3.2%, 4.5% at 0.12, 0.46, 1.03, 2.05, 4.8, 12.7 mU/l. The functional detection limit of the assay was 0.09 mU/l. Results for the free thyroxine method correlated well with the IMx (r = 0.91) and the equilibrium dialysis (r = 0.95) assay. Results for the thyrotropin method correlated well with the Tandem-TSH (r = 0.99) and the IMx (r = 0.99) assays. The euthyroid reference range was 11-23 pmol/l and 0.5-3.9 mU/l for free thyroxine and thyrotropin respectively. The free thyroxine assay was not influenced by changes in albumin or thyroxine binding globulin concentration but showed increases at oleic acid concentrations > 4 mmol/l. Spuriously elevated free thyroxine concentration were found in 4 patients, due to assay interference by antibodies in the serum. In a follow up study of 46 patients with non-thyroidal illness, serial measurements showed fluctuating free thyroxine and thyrotropin concentrations with abnormal results occurring in 34%. In a hospital setting, a wider range of free thyroxine (10-28 pmol/l) and thyrotropin (0.22-5.9 mU/l) concentration may be observed in patients who are clinically euthyroid. Abnormal thyroid function tests were however transient and follow up resolved most diagnostic problems.

Factors affecting the serum free thyroxine levels in hospitalized chronic geriatric patients

OBJECTIVE

Determination of whether nonthyroidal factors affect the diagnostic value of free thyroxine estimation in geriatric patients.

DESIGN

Survey.

PARTICIPANTS

A convenience sample of 381 non-selected, chronic, hospitalized geriatric patients over 60 years of age (I = relatively good health; II = relatively poor health; III = bad health; subgroups "sine therapia," ie, patients receiving no drugs that affect FT4) and 180 20-40 year old healthy persons.

MEASUREMENTS

Thyrotropin-releasing hormone test; thyrotropin (TSH); free thyroxine (FT4, measured in part by two parallel methods) estimation in a screening study; and thyroxine-binding globulin and thyroxine-binding-inhibitor activity measurements.

RESULTS

The normal FT4 ranges of the euthyroid geriatric (n = 210) and healthy young groups were similar. In the "sine therapia" euthyroid patients, FT4 decreased with age but increase with the severity of illness. High FT4 levels with non-suppressed TSH were more frequent in patients in poor and bad health. (I = 6/112; II = 14/140; III = 13/74; P < 0.01). The serum thyroxine-binding-inhibitor activity of euthyroid geriatric patients correlated with the severity of their clinical state (I = 6.22 +/- 5.65 (13); II = 7.40 +/- 4.33 (23); III = 10.04 +/- 5.50 (16) micrograms merthiolate equivalent/microL; ANOVA with log-transformed values: F(2.51) = 3.50, P < 0.05). The mean FT4 was higher in 36 heparin-treated patients (22.81 +/- 4.67 pmol/L) than in the 193 "sine therapia" patients (19.03 +/- 4.23 pmol/L; -P < 0.001). In a convenience subsample of 240 patients, a weak inverse correlation was found between FT4 and the thyroxine-binding globulin (r = -0.14, P < 0.02). Only 5/11 patients with low free thyroxine had hypothyroidism, while 11/46 patients with elevated free thyroxine had hyperthyroidism.

CONCLUSIONS

There is no need to modify the normal free thyroxine range for hospitalized geriatric patients. Clinical condition, drug treatment, and, to a lesser extent, age are factors that significantly affect the diagnostic value of FT4 in hospitalized chronic geriatric patients, decreasing the specificity of the test in diagnosing clinical hyper- and hypothyroidism.

Thyroid disease

Hyperthyroidism and hypothyroidism are common disorders in the elderly and may remain unrecognised until a patient presents with an apparently unrelated problem. The finding of an elevated level of thyroid stimulating hormone (TSH) with a normal serum thyroxine (T4) level represents "subclinical hypothyroidism", which does not necessarily require treatment. Iodine can precipitate hyperthyroidism in patients with autonomous thyroid tissue and the iodine-rich antiarrhythmic agent, amiodarone, may cause either hyperthyroidism or hypothyroidism. The metabolism and clearance of numerous therapeutic agents is altered when thyroid status is abnormal, so that dose adjustment may be necessary. In cardiac failure secondary to hyperthyroidism, great care must be taken in prescribing beta-blockers and diuretics; dosage of digitalis preparations may need to be increased. Thyroid replacement therapy can aggravate myocardial ischaemia and it may be appropriate to consider coronary artery bypass grafting before hypothyroidism is fully corrected. Antithyroid drugs, surgery and radioactive iodine all have a place in the treatment of hyperthyroidism in the elderly, depending on factors such as disease severity and the characteristics of the goitre. T4 may be given together with an antithyroid drug in a "block-replace" regimen.

Fatal nonthyroidal illness may impair nocturnal thyrotropin levels

PURPOSE

This study was undertaken to assess whether circadian variation of thyrotropin (TSH) is affected by the severity of a nonthyroidal illness.

PATIENTS AND METHODS

On the second day of admission to a medical intensive care unit, 20 consecutive patients with the major diagnosis of acute respiratory failure underwent TSH measurements at 8 A.M., 9 P.M., 11 P.M., and 1 A.M., with two sensitive assays.

RESULTS

Six patients died, five of whom had hypothyroxinemia (thyroxine [T4] less than 5.5 micrograms/dL) (83%) on the day of the study, whereas only three of the 14 survivors had low T4 (21%; p less than 0.05). Baseline 8 A.M. TSH measured with the two assays was similar in both groups and there was a progressive increase in TSH in survivors and a decrease in nonsurvivors at 9 P.M. and 11 P.M. However, the difference at these time points was not statistically significant. At 1 A.M., TSH levels were significantly lower among nonsurvivors (0.75 +/- 0.34 microU/mL with assay 1, and 0.7 +/- 0.4 microU/mL with assay 2) than in survivors (2.3 +/- 0.46 microU/mL with assay 1, and 2 +/- 0.5 microU/mL with assay 2; p less than 0.005; Wilcoxon test). Five of the nonsurvivors and none of the survivors had a suppressed 1 A.M. TSH level (p less than 0.001), suggesting a good correlation between suppressed 1 A.M. TSH and mortality. After exclusion of patients receiving drugs known to affect TSH levels (two nonsurvivors and four survivors), the same dissociation in TSH changes was observed, and significantly lower 1 A.M. TSH levels were observed in nonsurvivors than in survivors (0.13 +/- 0.08 microU/mL versus 2.7 +/- 0.6 microU/mL with assay 1; p less than 0.01). Cortisol levels were significantly higher only at 8 A.M. in nonsurvivors whether patients receiving drugs were included in the analysis (41.6 +/- 3.2 versus 28.4 +/- 2.7 micrograms/dL; p less than 0.01) or not (45.3 +/- 4.6 versus 30.5 +/- 3.6 micrograms/dL; p less than 0.01). At other times, cortisol levels were similar in both groups. The 24-hour TSH areas under the curve were also lower in nonsurvivors than in survivors whether patients receiving drugs known to affect TSH levels were included or not. However, cortisol areas under the curve were similar in both groups.

CONCLUSION

It is concluded that fatal illness is associated with a suppression of the late night TSH surge.

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 X 10(9)M-1) showed relative affinity triac approximately equal to T3 greater than T4. Drugs were studied over the concentration range 10(-3)-10(-7)M, approximating the known therapeutic total plasma concentrations, in competition with 125I-T3 0.1 nM, expressing inhibition as the percent decrement from maximum specific binding of 125I-T3 in drug vehicle (assay buffer or thanol 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 less than 0.02) and by diclofenac to 87.4 +/- 5.6% at 0.2 mM (p less than 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 less than 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 125I-T3 was increased over the range 0.1-0.5 nM.2+t

Clinical pharmacokinetic considerations in the treatment of increased intracranial pressure

Life-threatening increased intracranial pressure can be reversed by a variety of drugs. These compounds all have some disadvantages, producing rebound effects, severe coma or cardiovascular depression and electrolyte imbalance. However, reduction of intracranial pressure is a prerequisite for recovery and the benefits of treatment outweigh the risks. Dexamethasone is rapidly eliminated, the short half-life (about 3 hours) indicating that dosage intervals should be kept small. As yet, however, its therapeutic efficacy has not been clearly demonstrated. Therefore, an association between pharmacokinetics and pharmacodynamics cannot be established. Osmotic diuretics are the most widely used agents for reduction of intracranial pressure. Pharmacokinetics show a very close relationship to changes in serum osmolality, but there are large variations in the clearance. For the use of osmotics, the blood-brain barrier must be intact. Osmotic diuretics may lead to intracerebral oedema or to acute renal failure as serum osmolality increases. Considering the pharmacokinetics of each drug, and the dynamics of intracerebral pressure and osmolality, an intermittent, individually titrated dosage should be administered, with simultaneous monitoring of intracranial pressure. Frusemide (furosemide) can be used as an adjunct, to enhance the effect of osmotic diuretics. Its pharmacokinetics are limited by renal function, depending on age as well as on the extent of renal impairment. Altered renal elimination of concomitantly administered drugs, and electrolyte imbalances should be anticipated when diuretics are used. Barbiturates are certain to decrease intracranial pressure in humans by an as yet unknown mechanism. Their administration is recommended for patients that do not respond to conventional therapy. As barbiturates can result in deep coma, knowledge of their pharmacokinetics is of great importance for recovery. Following single doses, pentobarbitone has a relatively long elimination half-life (about 22 hours). However, after repeated administration for several days, its elimination may be enhanced due to autoinduction. Thiopentone kinetics are characterised by distribution and redistribution into deep peripheral compartments. Administration of high and frequent doses leads to considerably delayed recovery. This is not true for methohexitone, which shows comparable pharmacokinetics after single and multiple dose administration. Elimination depends on liver blood flow. Thus, recovery from methohexitone-coma is rapid. Rapid elimination is also an important characteristic of etomidate and alphaxalone/alphadolone, two non-barbiturate hypnotics.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of oral frusemide on diagnostic indices of thyroid function

We studied the acute effect of standard therapeutic doses of oral frusemide on indices of thyroid function in 34 hospital in-patients with congestive cardiac failure. A transient decrease in total T4, elevation in the T3 resin uptake and consequent increase in the free T4 index (FT4I) were seen 2-5 h after ingestion of frusemide at a chronic morning dosage of 80, 120 or 250 mg. The FT4I pre-vs post-frusemide values after 250 mg of drug were 109 +/- 12 vs 129 +/- 18 (P less than 0.05) after 120 mg 92 +/- 14 vs 119 +/- 12 (P less than 0.01), and after 80 mg 102 +/- 6 vs 112 +/- 4 (P less than 0.01) (mean +/- SEM). Similar increases in apparent free T4 measured by an analogue tracer assay (free T4 RIA sol, Henning, Berlin) were seen after frusemide. In a time course study, the major change in the T3 uptake 120 min after frusemide ingestion correlated with the change in serum frusemide concentration. When frusemide was added to serum in vitro its influence was greatest in methods that involved least dilution of serum. In two of the patients difficulty in clinical assessment of thyroid status was compounded by the effect of oral frusemide on FT4I. We conclude that oral frusemide may influence biochemical assessments of thyroid function in patients with congestive cardiac failure. It is necessary to consider the time interval between ingestion of high doses of oral frusemide and blood sampling in evaluating such results.

Recent advances in routine thyroid function testing

Thyroid function tests are one of the most common of endocrine laboratory investigations requested by general clinicians. The tests used therefore have to be efficient at identifying thyroid disease, monitoring treatment, and handling large numbers of tests. Recent advances in methodology have expanded both the range of in vitro thyroid function tests available and the techniques by which the well-established tests may be performed. This article reviews the methods and analytical and clinical performance of the routine tests currently available, concentrating particularly on the relatively new ones, and speculating on their role in strategies for the laboratory investigation of thyroid function.

Clinical and laboratory assessment of thyroid abnormalities

Clinical assessment of the patient with suspected thyroid disease remains an important part of the workup. Available laboratory tests of thyroid function include measurements of serum thyroid hormones and thyroid-stimulating hormone, titers of autoantibodies involved with Graves' disease and thyroiditis, and thyroid imaging and uptake techniques. The usefulness and limitations of each of these tests are reviewed.

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.