The effect of levothyroxine treatment in individuals with subclinical hypothyroidism on surrogate markers of atherosclerosis: a meta-analysis of randomized controlled trials

Cardiovascular outcomes in subclinical thyroid disease: an update

PURPOSE OF REVIEW

Subclinical thyroid disease is defined by a thyroid stimulating hormone (TSH) level outside of the normal range with normal circulating thyroid hormone levels. Excess adverse cardiovascular outcomes have been observed in certain patient populations with subclinical hypothyroidism (SCH) and hyperthyroidism (SCHr). The role of thyroid hormone and antithyroid treatments for subclinical thyroid disease remains debated.

RECENT FINDINGS

Cardiovascular disease appears to be a major mediator of all-cause mortality in patients with SCH, in particular those aged at least 60 years of age. In contrast, pooled clinical trial results did not find that levothyroxine reduced the incidence of cardiovascular events or mortality in this patient population. The association between SCHr and atrial fibrillation is well established; however, a 5-year follow-up of older patients with mild (TSH 0.1-0.4 mIU/l) SCHr found no increased incidence of atrial fibrillation. Separately, SCHr was associated with derangements in endothelial progenitor cell function that may underlie vascular disease independent from effects on cardiac function.

SUMMARY

The impact of treatment of subclinical thyroid disease on cardiovascular outcomes remains uncertain. Additional prospective and trial data are needed to evaluate treatment effects on cardiovascular outcomes in younger populations.

Meta-analytic evidence for increased low-grade systemic inflammation and oxidative stress in hypothyroid patients. Can levothyroxine replacement therapy mitigate the burden?

PURPOSE

This series of meta-analyses were aimed to elucidate the impact of hypothyroidism on low-grade systemic inflammation and oxidative stress assessed by C-reactive protein (CRP) and malondialdehyde (MDA) respectively; and to evaluate the effect of levothyroxine replacement therapy (LRT) on those outcomes.

METHODS

PubMed database and the key studies references were searched prior to March 3, 2020. Data on serum or plasma CRP and MDA levels in SHT (subclinical) and/or OHT (overt) hypothyroid patients and controls were extracted to compute overall standardized mean differences (SMD) by the random-effects model.

RESULTS

A total of 93 studies were entered into analyses and ten main meta-analyses were performed. OHT (SMD = 0.72 [0.39; 1.04], k = 35), SHT (SMD = 1.58 [0.78; 2.38], k = 56) and even mild SHT (TSH < 10 mU/L, SMD = 2.19 [0.02; 4.37], k = 13) proved to have a detrimental effect on CRP levels. LRT showed a favorable effect on CRP levels, particularly in OHT (SMD = -0.30 [-0.57; -0.02], k = 17). Increased levels of MDA were also found, especially in OHT (SMD = 2.49 [0.66; 4.31], k = 13). LRT may also improve MDA levels; however future studies would further validate the advantageous effect of LRT in hypothyroidism. Heterogeneity primarily originated from different study designs and geographic locations.

CONCLUSION

Overall, these meta-analyses reveal that screening for hs-CRP and MDA in hypothyroid patients as simple biomarkers of low-grade systemic inflammation and oxidative stress may become a useful tool to identify those at increased risk who may benefit most from early interventions.