Thyroid hormone and the heart

Subclinical Hypothyroidism and Lipid Metabolism: To Treat or Not to Treat?

Thyroid hormones have multiple complex effects on lipid synthesis and metabolism. These physiological actions are well documented in overt hypothyroidism where the elevated levels of total cholesterol, low density lipoporotein cholesterol and possibly triglycerides are reverted by levo-thyroxine therapy. Subclinical hypothyroidism, defined as elevated serum thyroid stimulating hormone in the presence of reference range of free thyroxine and free triiodothyronine concentrations, is a relatively frequent clinical conditions. Many clinical and epidemiological studies have evaluated lipid metabolism, markers of subclinical atherosclerosis and other cardiovascular risk factors in subclinical hypothyroidism as well as the need of replacement therapy in these patients. The available results are rather conflicting, with variable and inconclusive results. Moreover, no consensus still exists on the clinical significance and treatment of this mild form of thyroid failure. On the contrary, available evidences suggest that patients with plasma thyroid stimulating hormone levels above 10 mU/L should be treated with levo-thyroxine, since may have an increased risk of cardiovascular disease. However, the epidemiological evidences suggest being rather conservative in older people, since higher thyroid stimulating hormone is associated with lower risk of multiple adverse events in this population. In this review, we summarized the current evidences on the association between subclinical hypothyroidism and lipid metabolism and the effect of levo-thyroxine therapy on lipid parameters.

Cardiac myosin heavy chain gene regulation by thyroid hormone involves altered histone modifications

The antithetical regulation of cardiac α- and β-myosin heavy chain (MHC) genes by thyroid hormone (T(3)) is not well understood but appears to involve thyroid hormone interaction with its nuclear receptor and MHC promoters as well as cis-acting noncoding regulatory RNA (ncRNA). Both of these phenomena involve epigenetic regulations. This study investigated the extent that altered thyroid state induces histone modifications in the chromatin associated with the cardiac MHC genes. We hypothesized that specific epigenetic events could be identified and linked to cardiac MHC gene switching in response to a hypothyroid or hyperthyroid state. A hypothyroid state was induced in rats by propylthiouracil treatment (PTU), whereas a hyperthyroid (T(3)) was induced by T(3) treatment. The left ventricle was analyzed after 7 days for MHC pre-mRNA expression, and the chromatin was assessed for enrichment in specific histone modifications using chromatin immunoprecipitation quantitative PCR assays. At both the α-MHC promoter and the intergenic region, the enrichment in acetyl histone H3 at K9/14 (H3K9/14ac) and trimethyl histone H3 at K4 (H3K4me3) changed in a similar fashion. They were both decreased with PTU treatment but did not change under T(3), except at a location situated 5' to the antisense intergenic transcription start site. These same marks varied differently on the β-MHC promoter. For example, H3K4me3 enrichment correlated with the β-promoter activity in PTU and T(3) groups, whereas H3K9/14ac was repressed in the T(3) group but did not change under PTU. Histone H3K9me was enriched in chromatin of both the intergenic and α-MHC promoters in the PTU group, whereas histone H4K20me1 was enriched in chromatin of β-MHC promoter in the normal control and T(3) groups. Collectively, these findings provide evidence that specific epigenetic phenomena modulate MHC gene expression in altered thyroid states.

Effect of thyroid hormone supplementation on survival of euthyroid dogs with congestive heart failure due to systolic myocardial dysfunction: a double-blind, placebo-controlled trial

Nineteen euthyroid dogs of 12 breeds with echocardiographic signs of dilated cardiomyopathy (DCM) and radiographic and clinical signs of congestive heart failure (CHF) were evaluated in a randomised, double-blind, and placebo-controlled study. The dogs received either thyroxine or placebo as an adjunct to digoxin, furosemide and propranolol. The group assignment of individual dogs and serum concentrations of thyroid hormones remained unknown to owners and investigators during the entire study period. Dogs were evaluated clinically and with electrocardiography (ECG), thoracic radiography, echocardiography and measurement of total thyroxine (tT4) and thyroid stimulating hormone (TSH) before beginning of the trial, and then one week, 2 months, 6 months and yearly after initial examination, and, when applicable, at the time of euthanasia. End-point of the study was euthanasia (n = 17) due to severe congestive heart failure or sudden death (n = 2). Survival times ranged from 17 to 1030 days (median 187 days) in the placebo group, and from 18 to 1000 days (median 73 days) in the treatment group. There was no statistically significant difference in survival times between the treatment group and the placebo group (p = 0.46). Post mortem and histopathologic examinations revealed the attenuated wavy fiber type of DCM in 11 dogs, and myocardial infarcts, arteriosclerosis and chronic valvular disease in one dog. In conclusion, there was a wide range in survival times of dogs treated with digoxin, furosemide and propranolol. Adding thyroid hormones to the treatment did not significantly influence survival.

Thyroid function is associated with presence and severity of coronary atherosclerosis

BACKGROUND

Overt hypothyroidism has been found to be associated with cardiovascular disease. Moreover, subclinical hypothyroidism is a strong indicator of risk for aortic atherosclerosis and myocardial infarction.

HYPOTHESIS

We hypothesized that variation of thyroid function within the normal range may influence the presence and severity of coronary atherosclerosis.

METHODS

We studied a total of 100 consecutive men and women (59 men, 41 women, age 63.7 +/- 11.0 years) who underwent coronary angiography. Blood was tested for serum thyrotropin concentrations and for free tri-iodothyronine and free thyroxine concentrations. In addition to the assessment of thyroid function, conventional risk factors for coronary artery disease (CAD), clinical characteristics, serum lipid levels, fasting total homocysteine, and angiographic results of coronary artery assessment were obtained. Two experienced cardiologists blinded to clinical and laboratory data reviewed the cinefilms. The severity of CAD was scored as 0 for those with smooth normal epicardial coronary arteries, 0.5 for plaquing (< 50% diameter stenosis), and 1, 2, or 3 for those with single-, double-, or triple-vessel epicardial coronary artery stenosis of > 50%, respectively.

RESULTS

The severity of CAD was scored as 0, 0.5, 1, 2, and 3 in 14, 26, 25, 22, and 13 patients, respectively. Higher levels of serum-free thyroid hormone concentrations were associated with decreased severity of coronary atherosclerosis. Serum-free tri-iodothyronine was 2.99 +/- 0.33 pg/ml in patients with a CAD severity score of 0 to 1 and 2.74 +/- 0.49 pg/ml in patients with CAD severity scores of 2 and 3 (p < 0.01). Moreover, serum-free thyroxine concentrations showed a trend toward higher levels in patients with CAD severity score 0 to 1 compared with patients with CAD severity scores 2 and 3 (11.65 +/- 1.87 pg/ml vs. 10.9 +/- 2.3 pg/ml; p = 0.09). Higher levels of serum thyrotropin concentrations were associated with increased severity of coronary atherosclerosis (1.37 +/- 1.02 mU/l vs. 1.98 +/- 2.13 mU/l in patients with CAD severity score 0 to 1 versus CAD severity scores 2 and 3; p = 0.049). When grouped into three subsets according to their serum free tri-iodothyronine levels (< 2.79, 2.8 to 3.09, and +/- 3.1 pg/ml), the prevalence of CAD scores 2 and 3 was significantly higher in the subset of patients with low serum free tri-iodothyronine levels (48.5%) than in the subsets of patients with medium or high tri-iodothyronine concentrations (32.25 and 25%, respectively, p for trend < 0.05).

CONCLUSION

These data in patients referred for coronary angiography suggest that variation of thyroid function within the statistical normal range may influence the presence and severity of coronary atherosclerosis.

Impact of hyperthyroidism and its correction on vascular reactivity in humans

BACKGROUND

Although thyroid hormone (TH) exerts relevant effects on the cardiovascular system, it is unknown whether TH also regulates vascular reactivity in humans. Methods and Results- We studied 8 patients with hyperthyroidism, basally (H) and 6 months after euthyroidism was restored by methimazole (EU). Thirteen healthy subjects served as control subjects (C). We measured forearm blood flow (FBF) by strain-gauge plethysmography during intrabrachial graded infusion of acetylcholine, sodium nitroprusside (SNP), norepinephrine, and L-NMMA (inhibitor of NO synthesis). Basal FBF (in mL. dL(-1). min(-1)) was markedly higher in H than in C (5.8+/-1.2 and 1.9+/-0.1, respectively; P<0.001) and was close to normal in EU (2.6+/-0.3, P<0.01 versus H). During acetylcholine infusion, FBF increased much more in H (+33+/-5) than in C (+14+/-3, P<0.01 versus H) and in EU (+20+/-5, P=0.01 versus H and P=NS versus C). In contrast, the response to SNP infusion was comparable in the patients and control subjects. During norepinephrine infusion, the fall in FBF was much more pronounced in H (-6+/-1) than in C (-0.7+/-0.3, P<0.005 versus H) and in EU (-1.5+/-0.3, P<0.01 versus H). Finally, inhibition of NO synthesis by L-NMMA decreased FBF by 2.8+/-0.6, 0.61+/-0.7, and 1.4+/-0.3 in H, C, and EU, respectively (H versus C and EU, P<0.05).

CONCLUSIONS

In hyperthyroidism, (1) the marked basal vasodilation is largely accounted for by excessive endothelial NO production, (2) vascular reactivity is exaggerated because of enhanced sensitivity of the endothelial component, (3) the vasoconstrictory response to norepinephrine is potentiated, and (4) this abnormal vascular profile is corrected when euthyroidism is restored by medical therapy. The data demonstrate that vascular endothelium is a specific target of TH.

Effects of dilated cardiomyopathy on the renin-angiotensin-aldosterone system, atrial natriuretic peptide activity, and thyroid hormone concentrations in dogs

OBJECTIVE

To evaluate the effect of dilated cardiomyopathy (DCM) on activity of the renin-angiotensin-aldosterone system (RAAS), the N-terminal fragment of proatrial natriuretic peptide (NT-proANP), and thyroid hormone concentrations in dogs.

ANIMALS

15 dogs with clinical signs of DCM, 15 dogs without clinical signs of DCM, and 15 age-, breed-, and sex-matched control dogs.

PROCEDURE

Physical examinations, thoracic radiography, ECG, and echocardiography were performed on all dogs, and blood and urine samples were collected.

RESULTS

Plasma renin activity (PRA), plasma aldosterone concentration (PAC), urine aldosterone-to-creatinine ratio, and NT-proANP concentrations were significantly increased in dogs with clinical signs of DCM, compared with dogs without clinical signs and control dogs. Thyroid-stimulating hormone and total thyroxine concentrations did not differ significantly among groups; however, free thyroxine (FT4) concentrations were significantly decreased in dogs with clinical signs of DCM, compared with control dogs and DCM-dogs without clinical signs. Concentrations of PRA, PAC, FT4, and urine aldosterone-to-creatinine ratio were significantly correlated, whereas plasma concentrations of NT-proANP only correlated with FT4 concentration.

CONCLUSION AND CLINICAL RELEVANCE

In dogs with clinical signs of DCM, increased concentrations of components of the RAAS were associated with increased concentrations of NT-proANP Analysis of the neurohormonal system may aid in identification of clinical stages of DCM for groups of dogs, but the range is too great and there are too many dogs that have neurohormonal concentrations within reference ranges to assess dogs on an individual basis.

The p23 molecular chaperones act at a late step in intracellular receptor action to differentially affect ligand efficacies

Multiple molecular chaperones, including Hsp90 and p23, interact with members of the intracellular receptor (IR) family. To investigate p23 function, we compared the effects of three p23 proteins on IR activities, yeast p23 (sba1p) and the two human p23 homologs, p23 and tsp23. We found that Sba1p was indistinguishable from human p23 in assays of seven IR activities in both animal cells and in yeast; in contrast, certain effects of tsp23 were specific to that homolog. Transcriptional activation by two IRs was increased by expression of any of the p23 species, whereas activation by five other IRs was decreased by Sba1p or p23, and unaffected by tsp23. p23 was expressed in all tissues examined except striated and cardiac muscle, whereas tsp23 accumulated in a complementary pattern; hence, p23 proteins might contribute to tissue-specific differences in IR activities. Unlike Hsp90, which acts on IR aporeceptors to stimulate ligand potency (i.e., hormone-binding affinity), p23 proteins acted on IR holoreceptors to alter ligand efficiencies (i.e., transcriptional activation activity). Moreover, the p23 effects developed slowly, requiring prolonged exposure to hormone. In vitro, p23 interacted preferentially with hormone–receptor–response element ternary complexes, and stimulated receptor–DNA dissociation. The dissociation was reversed by addition of a fragment of the GRIP1 coactivator, suggesting that the two reactions may be in competition in vivo. Our findings suggest that p23 functions at one or more late steps in IR-mediated signal transduction, perhaps including receptor recycling and/or reversal of the response.