Cardiac function in thyroid disease: clinical features and management considerations

Low Thyroid Function Leads to Cardiac Atrophy With Chamber Dilatation, Impaired Myocardial Blood Flow, Loss of Arterioles, and Severe Systolic Dysfunction

Background— Although thyroid dysfunction has been linked to heart failure, it is not clear whether hypothyroidism alone can cause heart failure.

Methods and Results— Hypothyroidism was induced in adult rats by treatment with 0.025% propylthiouracil (PTU) for 6 weeks (PTU-S) and 1 year (PTU-L). Echocardiographic measurements, left ventricular (LV) hemodynamics, isolated myocyte length (KOH method), myocardial blood flow (fluorescent microspheres), arteriolar morphometry, and gene expression (Western blot) were determined. Heart weight, heart rate, LV systolic blood pressure, LV ejection fraction, LV fractional shortening, and systolic wall thickness were reduced in PTU-S and PTU-L rats. LV internal diameter in systole increased by 40% in PTU-S and 86% in PTU-L. LV internal dimension in diastole was increased in PTU-S and PTU-L rats, but only PTU-L rats showed a significant increase in myocyte length due to series sarcomere addition. Resting and maximum (adenosine) myocardial blood flow were reduced in both PTU-S and PTU-L rats. Impaired blood flow was due to a large reduction in arteriolar length density and small arterioles in PTU-S and PTU-L ( P <0.05 or greater for all of the above comparisons). Expression of sarcoplasmic/endoplasmic reticulum Ca 2+ -ATPase (SERCA)-2a and α-myosin heavy chain were reduced in hypothyroidism, whereas phospholamban and β-myosin heavy chain were increased.

Conclusions— Hypothyroidism led to severe, progressive systolic dysfunction and increased chamber diameter/wall thickness ratio despite a reduction in cardiac mass. Chamber dilatation in PTU-L rats was due to series sarcomere addition, typical of heart failure. Hypothyroidism resulted in impaired myocardial blood flow due to a dramatic loss of arterioles. Thus, we have identified 2 important new mechanisms by which low thyroid function may lead to heart failure.

Thyroid hormone induces cardiac myocyte hypertrophy in a thyroid hormone receptor alpha1-specific manner that requires TAK1 and p38 mitogen-activated protein kinase

Alterations in TR [thyroid hormone (TH) receptor]1 isoform expression have been reported in models of both physiologic and pathologic cardiac hypertrophy as well as in patients with heart failure. In this report, we demonstrate that TH induces hypertrophy as a direct result of binding to the TRalpha1 isoform and, moreover, that overexpression of TRalpha1 alone is also associated with a hypertrophic phenotype, even in the absence of ligand. The mechanism of TH and TRalpha1-specific hypertrophy is novel for a nuclear hormone receptor and involves the transforming growth factor beta-activated kinase (TAK1) and p38. Mitigating TRalpha1 effects, both TRalpha2 and TRbeta1 attenuate TRalpha1-induced myocardial growth and gene expression by diminishing TAK1 and p38 activities, respectively. These findings refine our previous observations on TR expression in the hypertrophied and failing heart and suggest that manipulation of thyroid hormone signaling in an isoform-specific manner may be a relevant therapeutic target for altering the pathologic myocardial program.

Thyroid hormone stimulates myoglobin gene expression in rat cardiac muscle

T3 increases the heart activity, O2 consumption and the reactive O2 species production. Myoglobin (Mb) is highly expressed in the heart, where it facilitates O2 diffusion, mitochondrial respiration, and scavenges reactive O2 species. Here we investigate, by dose-response (0.3-100 microg/100 g BW, i.p., 5 days) and time-course studies (100 microg/100 g BW, i.v., from 0.5 to 24h), whether T3 affects the Mb mRNA and protein expression in atrium (A) and ventricle (V), by Northern and Western blot. We show that the Mb gene is controlled by T3 in A and V, as indicated by Mb mRNA and protein content decrease in thyroidectomized (Tx) rats, and restoration by T3 treatment. In the A, the different doses of T3 induced the Mb mRNA and protein recovery to the euthyroid levels; in the time-course study, this occurred only with the protein levels. In the V, T3 progressively increased the Mb mRNA above the euthyroid levels at a dose of 25 microg/100g BW; higher doses decreased it to the euthyroid levels. Mb protein increased only to the euthyroid levels at all T3 doses injected. The time-course study showed a progressive increase in the ventricular Mb mRNA and protein, which exceeded the euthyroid levels from 6 to 24h, and at 2 and 6 h of the T3 treatment, respectively. We conclude that heart Mb gene expression is influenced by thyroid status.

Hypothyroidism as a risk factor for cardiovascular disease

The cardiovascular risk in patients with hypothyroidism is related to an increased risk of functional cardiovascular abnormalities and to an increased risk of atherosclerosis. The pattern of cardiovascular abnormalities is similar in subclinical and overt hypothyroidism, suggesting that a lesser degree of thyroid hormone deficiency may also affect the cardiovascular system. Hypothyroid patients, even those with subclinical hypothyroidism, have impaired endothelial function, normal/depressed systolic function, left ventricular diastolic dysfunction at rest, and systolic and diastolic dysfunction on effort, which may result in poor physical exercise capacity. There is also a tendency to increase diastolic blood pressure as a result of increased systemic vascular resistance. All these abnormalities regress with L-T4 replacement therapy. An increased risk for atherosclerosis is supported by autopsy and epidemiological studies in patients with thyroid hormone deficiency. The "traditional" risk factors are hypertension in conjunction with an atherogenic lipid profile; the latter is more often observed in patients with TSH >10 mU/L. More recently, C-reactive protein, homocysteine, increased arterial stiffness, endothelial dysfunction, and altered coagulation parameters have been recognized as risk factors for atherosclerosis in patients with thyroid hormone deficiency. This constellation of reversible cardiovascular abnormalities in patient with TSH levels <10 mU/L indicate that the benefits of treatment of mild thyroid failure with appropriate doses of L-thyroxine outweigh the risk.

Thyroid hormone supplementation for the prevention of morbidity and mortality in infants undergoing cardiac surgery

BACKGROUND

Paediatric studies have demonstrated that cardiopulmonary bypass is associated with a decline in thyroid hormone levels. Adult patients who undergo open heart surgery and receive triiodothyronine supplementation have demonstrated a dose-dependent increase in cardiac output which has been associated with an improved clinical outcome. Thyroid hormone supplementation in infants may also reduce post-operative morbidity and mortality.

OBJECTIVES

To determine if peri-operative thyroid hormone supplementation or replacement in infants undergoing cardiac surgery on cardiopulmonary bypass improves post-operative and longer term morbidity and mortality.

SEARCH STRATEGY

The standard search strategy of the Cochrane Neonatal Review Group was used. This included searches of The Oxford Database of Perinatal Trials, MEDLINE (1966 - December 2003), EMBASE (1980 - December 2003), CINAHL (1982 - December 2003), The Cochrane Central Register of Controlled Trials (The Cochrane Library, Issue 2, 2003), previous reviews including cross references, abstracts, conferences, symposia proceedings, expert informants and journal handsearching in the English language.

SELECTION CRITERIA

All trials using random allocation to peri-operative thyroid hormone therapy (supplementation or replacement) compared to control (placebo or no therapy) in infants (birth to one year of age) undergoing cardiac surgery requiring cardiopulmonary bypass. Thyroid hormone therapy must be tri-iodothyronine.

DATA COLLECTION AND ANALYSIS

Primary clinical outcomes included measures of post-operative morbidity and mortality. The standard methods of the Cochrane Neonatal Review Group were used in the assessment of trial quality. Treatment effects were expressed using relative risk (RR) and mean difference (MD).

MAIN RESULTS

Two very small studies were identified that tested peri-operative thyroid hormone supplementation or replacement in infants aged less than one year undergoing cardiac surgery (Chowdhury 2001; Portman 2000). In the Chowdhury 2001 study, a subgroup of nine neonates was eligible for this review. No deaths occurred during either study. Chowdhury 2001 found no significant effect of peri-operative thyroid hormone supplementation in neonates on either length of hospital stay or duration of mechanical ventilation. Portman 2000 found no significant difference in dopamine requirements for the treatment versus control groups for the first 24 hours post operatively, while in the Chowdhury neonatal subgroup, inotrope requirements were significantly lower in the treatment group. Portman 2000 reported significant differences between the two groups at 1 and 24 hours post operatively for free T3 and at 1 hour post operatively for total T3 levels. Total T4 levels showed no significant difference between groups, either pre-cardiopulmonary bypass or up to 72 hours post operatively.

REVIEWERS' CONCLUSIONS

At present, there is a lack of evidence concerning the effects of tri-iodothyronine supplementation in infants undergoing cardiac surgery. Further randomised controlled trials which include sufficiently large subject numbers in a variety of different age strata (neonates, infants and older children) need to be undertaken.

Design of thyroid hormone receptor antagonists from first principles

It is desirable to obtain TR antagonists for treatment of hyperthyroidism and other conditions. We have designed TR antagonists from first principles based on TR crystal structures. Since agonist ligands are buried in the fold of the TR ligand binding domain (LBD), we reasoned that ligands that resemble agonists with large extensions should bind the LBD, but would prevent its folding into an active conformation. In particular, we predicted that extensions at the 5' aryl position of ligand should reposition helix (H) 12, which forms part of the co-activator binding surface, and thereby inhibit TR activity. We have found that some synthetic ligands with 5' aryl ring extensions behave as antagonists (DIBRT, NH-3), or partial antagonists (GC-14, NH-4). Moreover, one compound (NH-3) represents the first potent TR antagonist with nanomolar affinity that also inhibits TR action in an animal model. However, the properties of the ligands also reveal unexpected aspects of TR behavior. While nuclear receptor antagonists generally promote binding of co-repressors, NH-3 blocks co-activator binding and also prevents co-repressor binding. More surprisingly, many compounds with extensions behave as full or partial agonists. We present hypotheses to explain both behaviors in terms of dynamic equilibrium of H12 position.

Nongenomic cardiovascular effects of triiodothyronine in euthyroid male volunteers

T(3) has been shown to exert cardiovascular effects. These effects have not yet been defined with regard to the mode of action (nongenomic vs. genomic) and with regard to an interaction with the adrenergic system in humans. To address these issues we conducted a randomized, double blind, 6-fold cross-over trial in 18 healthy male volunteers. After pretreatment with the beta-agonist dobutamine, the beta-blocking agent esmolol, or placebo (0.9% NaCl), 100 microg T(3) or placebo were injected. Primary target variables were systemic vascular resistance (SVR) and cardiac output (CO) within 45 min after injection of T(3) vs. placebo after placebo pretreatment. Sympatho-vagal balance was assessed by measurement of heart rate variability. T(3) caused a lower SVR and a higher CO than placebo (P < 0.001) after pretreatment with placebo. An increased low frequency (LF)/high frequency (HF) ratio (power in LF/power in HF band) after T(3) compared with placebo (P = 0.004) suggests an increase in sympathetic tone. After pretreatment with dobutamine, the effects of T(3) on SVR and CO were abolished, and the effect on LF/HF ratio was reversed. After pretreatment with esmolol, the effects on SVR and LF/HF ratio were reversed. Our data show, for the first time, nongenomic cardiovascular effects of T(3) in humans.

Selective modulation of thyroid hormone receptor action

Thyroid hormones have some actions that might be useful therapeutically, but others that are deleterious. Potential therapeutically useful actions include those to induce weight loss and lower plasma cholesterol levels. Potential deleterious actions are those on the heart to induce tachycardia and arrhythmia, on bone to decrease mineral density, and on muscle to induce wasting. There have been successes in selectively modulating the actions of other classes of hormones through various means, including the use of pharmaceuticals that have enhanced affinities for certain receptor isoforms. Thus, there is reason to pursue selective modulation of thyroid hormone receptor (TR) function, and several agents have been shown to have some beta-selective, hepatic selective and/or cardiac sparring activities, although development of these was largely not based on detailed understanding of mechanisms for the specificity. The possibility of selectively targeting the TRbeta was suggested by the findings that there are alpha- and beta-TR forms and that the TRalpha-forms may preferentially regulate the heart rate, whereas many other actions of these hormones are mediated by the TRbeta. We determined X-ray crystal structures of the TRalpha and TRbeta ligand-binding domains (LBDs) complexed with the thyroid hormone analog 3,5,3'-triiodithyroacetic acid (Triac). The data suggested that a single amino acid difference in the ligand-binding cavities of the two receptors could affect hydrogen bonding in the receptor region, where the ligand's 1-position substituent fits and might be exploited to generate beta-selective ligands. The compound GC-1, with oxoacetate in the 1-position instead of acetate as in Triac, exhibited TRbeta-selective binding and actions in cultured cells. An X-ray crystal structure of the GC-1-TRbeta LBD complex suggests that the oxoacetate does participate in a network of hydrogen bonding in the TR LBD polar pocket. GC-1 displayed actions in tadpoles that were TRbeta-selective. When administered to mice, GC-1 was as effective in lowering plasma cholesterol levels as T(3), and was more effective than T(3) in lowering plasma triglyceride levels. At these doses, GC-1 did not increase the heart rate. GC-1 was also less active than T(3) in modulating activities of several other cardiac parameters, and especially a cardiac pacemaker channel such as HCN-2, which may participate in regulation of the heart rate. GC-1 showed intermediate activity in suppressing plasma thyroid stimulating hormone (TSH) levels. The tissue/plasma ratio for GC-1 in heart was also less than for the liver. These data suggest that compounds can be generated that are TR-selective and that compounds with this property and/or that exhibit selective uptake, might have clinical utility as selective TR modulators.

Pharmacokinetic evaluation of triiodothyronine supplementation in children after modified Fontan procedure

BACKGROUND

Triiodothyronine (T(3)) supplementation may be a useful adjunct in the management of patients after cardiopulmonary bypass. Limited data are available regarding the use and pharmacokinetics of T(3) in children. The present study was performed to evaluate T(3) pharmacokinetics in a cohort of children undergoing the modified Fontan procedure.

METHODS AND RESULTS

A total of 28 patients were enrolled in this randomized, prospective study. The patients were divided into 4 groups: 1 group received a placebo and 3 groups received intravenous T(3) at dosages of 0.4, 0.6, and 0.8 microg/kg, respectively. All 28 patients survived their operative procedures. Two patients developed low cardiac output, and 3 patients had pleural effusions. The median length of hospital stay was 7 days. The mean free T(3) level was 316+/-67 pg/dL after then administration of a placebo. Patients who received T(3) had mean peak free T(3) levels of 972+/-88, 1351+/-299, and 1869+/-281 pg/dL for the dosages of 0.4, 0.6, and 0.8 microg/kg, respectively. The calculated half-life of T(3) was 7 hours.

CONCLUSIONS

The half-life of intravenous T(3) in children is approximately one-third of that reported for adults. These results provide a framework for studying the efficacy of T(3) supplementation in children undergoing open-heart surgery.

Thyroid hormone and cardiovascular disease

Thyroid hormone directly affects the heart and peripheral vascular system. The hormone can increase myocardial inotropy and heart rate and dilate peripheral arteries to increase cardiac output. An excessive deficiency of thyroid hormone can cause cardiovascular disease and aggravate many preexisting conditions. In severe systemic illness and after major surgical procedures changes in thyroid function can occur, leading to the "euthyroid sick syndrome." Patients will have normal or decreased levels of T4, decreased free and total T3, and usually normal levels of thyroid stimulating hormone. This syndrome may be an adaptive response to systemic illness that usually will revert to normal without hormone supplementation as the illness subsides. Recently, however, many investigators have explored the benefits of thyroid hormone supplementation in those diseases associated with euthyroid sick syndrome. Thyroid hormone's effects on the cardiovascular system make it an attractive therapy for those patients with impaired hemodynamics and low T3. Thyroid hormone has also been considered a treatment for patients with congestive heart failure, for patients undergoing cardiopulmonary bypass and heart transplantation, and for patients with hyperlipidemia. At present there is no evidence suggesting a favorable treatment outcome using thyroid hormone supplementation for any systemic condition except in those patients with documented hypothyroidism.

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.

Reduction in triiodothyronine levels following modified Fontan procedure

Diminished cardiac function is a common manifestation following the modified Fontan procedure. Since thyroid hormone has important effects on cardiovascular function, the present study was undertaken to evaluate changes in thyroid hormone levels following this operation. A control group consisting of children undergoing open heart procedures other than a Fontan procedure was also evaluated. Serum total and free triiodothyronine (T3), total and free thyroxine (T4), thyroid stimulating hormone (TSH), and thyroglobulin were measured by immunoassays. The Fontan group demonstrated an initial increase in free T4, while free T3, total T3, total T4, TSH, and thyroglobulin were reduced. Over the subsequent days, free T4 decreased to below the preoperative value. By the fifth and eighth postoperative days, free T3, total T3, free T4, and total T4 remained reduced, while TSH and thyroglobulin began increasing toward the preoperative levels. The control group also demonstrated decreases in free T3 and TSH. However, these values had returned to baseline by the fifth postoperative day. The results indicate that children undergoing open heart surgery have suppression of the pituitary-thyroid axis, and that this is prolonged in patients undergoing Fontan procedure. The decreased levels of T3 following Fontan procedure may have adverse effects on the recovery of patients undergoing this operation.