Effect of oral frusemide on diagnostic indices of thyroid function

Approach to the Patient With Raised Thyroid Hormones and Nonsuppressed TSH

Measurement of free thyroid hormones (THs) and thyrotropin (TSH) using automated immunoassays is central to the diagnosis of thyroid dysfunction. Using illustrative cases, we describe a diagnostic approach to discordant thyroid function tests, focusing on entities causing elevated free thyroxine and/or free triiodothyronine measurements with nonsuppressed TSH levels. Different types of analytical interference (eg, abnormal thyroid hormone binding proteins, antibodies to iodothyronines or TSH, heterophile antibodies, biotin) or disorders (eg, resistance to thyroid hormone β or α, monocarboxylate transporter 8 or selenoprotein deficiency, TSH-secreting pituitary tumor) that can cause this biochemical pattern will be considered. We show that a structured approach, combining clinical assessment with additional laboratory investigations to exclude assay artifact, followed by genetic testing or specialized imaging, can establish a correct diagnosis, potentially preventing unnecessary investigation or inappropriate therapy.

Relationship of Preoperative Thyroid Dysfunction to Clinical Outcomes in Pediatric Cardiac Surgery

CONTEXT

Thyroid function may be assessed in children before cardiac surgery because of concerns that hypothyroidism or thyrotoxicosis might adversely affect cardiac function perioperatively. However, the relationship between preoperative thyroid dysfunction and surgical outcomes is unknown.

OBJECTIVE

Determine the relationship between preoperative thyroid dysfunction and outcomes of pediatric cardiac surgery.

METHODS

Retrospective cohort study (January 2005 to July 2019).

SETTING

Academic pediatric hospital.

PATIENTS

All patients <19 years old who underwent cardiac surgery with cardiopulmonary bypass and had thyrotropin (TSH) measured within 14 days preoperatively. Exclusion criteria included neonates (≤30 days), preoperative extracorporeal life support, salvage operations, or transplantation procedures.

MAIN OUTCOME MEASURES

Subjects were stratified by preoperative TSH concentration (mIU/L): low (<0.5), normal (0.5-5), mildly high (5.01-10), or moderately high (>10). Outcomes were compared among subjects with normal TSH (control) and each group with abnormal TSH concentrations. The primary outcome was 30-day mortality. Secondary outcomes included time to extubation, intensive care unit and hospital length of stay, and operative complications.

RESULTS

Among 592 patients analyzed, preoperative TSH was low in 15 (2.5%), normal in 347 (58.6%), mildly high in 177 (29.9%), and moderately high in 53 (9.0%). Free thyroxine was measured in 77.4% of patients and was low in 0 to 4.4% of subjects, with no differences among TSH groups. Thirty-day mortality was similar among TSH groups. There were no differences in any secondary outcome between patients with abnormal TSH and patients with normal TSH.

CONCLUSION

Preoperative mild to moderate subclinical hypothyroidism was not associated with adverse postoperative outcomes in children undergoing cardiopulmonary bypass procedures.

Pitfalls in the measurement and interpretation of thyroid function tests

Thyroid function tests (TFTs) are amongst the most commonly requested laboratory investigations in both primary and secondary care. Fortunately, most TFTs are straightforward to interpret and confirm the clinical impression of euthyroidism, hypothyroidism or hyperthyroidism. However, in an important subgroup of patients the results of TFTs can seem confusing, either by virtue of being discordant with the clinical picture or because they appear incongruent with each other [e.g. raised thyroid hormones (TH), but with non-suppressed thyrotropin (TSH); raised TSH, but with normal TH]. In such cases, it is important first to revisit the clinical context, and to consider potential confounding factors, including alterations in normal physiology (e.g. pregnancy), intercurrent (non-thyroidal) illness, and medication usage (e.g. thyroxine, amiodarone, heparin). Once these have been excluded, laboratory artefacts in commonly used TSH or TH immunoassays should be screened for, thus avoiding unnecessary further investigation and/or treatment in cases where there is assay interference. In the remainder, consideration should be given to screening for rare genetic and acquired disorders of the hypothalamic-pituitary-thyroid (HPT) axis [e.g. resistance to thyroid hormone (RTH), thyrotropinoma (TSHoma)]. Here, we discuss the main pitfalls in the measurement and interpretation of TFTs, and propose a structured algorithm for the investigation and management of patients with anomalous/discordant TFTs.

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'.

The nonthyroidal illness syndrome

This article briefly summarizes thyroid function alterations generally seen in the euthyroid sick syndrome, provides an overview of specific thyroidal adaptations during several clinical conditions and secondary to specific pharmacologic agents, and discusses the current controversy in thyroid hormone treatment of nonthyroidal illness.

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.

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.

Increased levothyroxine requirements presenting as "inappropriate" TSH secretion syndrome in a patient with nephrotic syndrome

Patients with primary thyroid failure on levothyroxine (LT4) replacement who develop nephrotic syndrome (NS) may rarely present with an increase in LT4 requirements. In this report, we describe a patient with thyroid failure following radioactive iodine ablation for Graves' disease who required an escalation of LT4 doses following the onset of NS. The case presented with disproportionately elevated TSH levels in the presence of normal (or slightly subnormal) thyroid hormone levels, thus, masquerading as a state of "inappropriate" TSH secretion. This pattern of extreme dysregulation in thyroid function indices due to urinary loss of thyroid hormones has not been previously described in NS, and, therefore, extends the spectrum of endocrine manifestations of NS.

Effect of loop diuretics and nonsteroidal antiinflammatory drugs on thyrotropin release by rat anterior pituitary cells in vitro

The close inverse-feedback relationship between serum free thyroxine (T4) and thyrotropin (TSH) is altered in some patients receiving therapeutic doses of drugs such as furosemide, fenclofenac, and diphenylhydantoin. We therefore examined the effect of nonsteroidal antiinflammatory drugs (NSAID), diuretics, and diphenylhydantoin on TSH release in rat anterior pituitary cells in primary culture. TSH content of the culture medium was measured at 22 hours at 37 degrees C either with or without thyrotropin-releasing hormone ([TRH] 10 nmol/L) in medium containing 0.5% bovine serum albumin. The mean basal TSH release by pituitary cells was 6.2 +/- 1.2 ng/mL (n = 10) and was not influenced by unlabeled triiodothyronine ([T3] 100 nmol/L) or any of the drugs tested at < or = 400 mumol/L, except ethyacrynic acid. TRH 10 nmol/L increased mean TSH release by 346% +/- 95% (n = 10). T3 1 and 100 nmol/L inhibited TRH-stimulated TSH release by 24% and 31%, respectively (P < .001), whereas TRH-stimulated TSH release was inhibited by 100 mumol/L meclofenamic acid (29%), fenclofenac (28%), furosemide (24%), and diphenylhydantoin (48%) (P < .001 v TRH alone). Meclofenamic acid and furosemide (100 mumol/L) did not significantly alter the inhibitory effect of T3 1 nmol/L on TRH-stimulated TSH release. These in vitro studies suggest that meclofenamic acid, fenclofenac, furosemide, and diphenylhydantoin could influence TSH release by attenuating the TSH response to TRH. This effect may influence T4-TSH relationships when these agents are used in vivo.

A naturally occurring furan fatty acid enhances drug inhibition of thyroxine binding in serum

We studied the thyroxine (T4)-displacing effects of a naturally occurring, highly albumin-bound furanoid acid that accumulates in serum in renal failure to concentrations in excess of 0.2 mmol/L. This substance, 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF), has been shown to displace acidic drugs from albumin binding. The effects of CMPF on ligand binding were assessed in the following systems: (1) T4 binding to T4-binding globulin (TBG) and transthyretin (TTR), (2) T4 binding in undiluted serum, (3) T4-displacing potency of fenclofenac, furosemide, diflunisal, and aspirin in undiluted serum, (4) serum binding of [14C]-drug preparations, and (5) serum binding of [14C]-oleic acid. CMPF had a minor direct effect on T4 binding to TBG comparable in relative affinity to that of aspirin, ie, almost 7 orders of magnitude less than T4 itself. CMPF alone at a concentration of 0.3 mmol/L, which produced only a 10% to 14% increase in free T4 augmented the T4-displacing effects of high therapeutic concentrations of the various drugs in undiluted serum as follows: furosemide by 180%, fenclofenac by 160%, diflunisal by 130%, and aspirin by 40%. In the presence of fenclofenac, increments of CMPF from 0.075 to 0.3 mmol/L progressively augmented the T4-displacing effect of this drug, associated with a progressive increase in its calculated free concentration. CMPF also inhibited the binding of [14C]-oleic acid, suggesting that in some situations CMPF could also indirectly influence thyroid hormone binding by increasing the unbound concentration of nonesterified fatty acids (NEFA), as previously described.(ABSTRACT TRUNCATED AT 250 WORDS)

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