Background: Subclinical hypothyroidism (SCH) is defined as an elevation in thyroid stimulating hormone (TSH), in the absence of reductions in free thyroxine (FT4). An estimated 4.6 to 8.6 million Americans meet the criteria for SCH, but are without a significant symptom burden. Carotid artery intima media thickness (CIMT) assessment with ultrasound technology is a non-invasive modality to assess atherosclerosis and cardiovascular health. Research has demonstrated increases in CIMT in subjects with overt hypothyroidism at baseline; however, following levothyroxine therapy, CIMT was reduced. The objective of this literature review is to determine if levothyroxine treatment results in CIMT reduction in adult subjects with subclinical hypothyroidism.
Methods: An extensive literature search was performed using MEDLINE, CINAHL, Google Scholar and ISI Web of Science. Studies were selected for analysis based on the following inclusion/ exclusion criteria: published in the English language, human subjects, since 1998 and assessing subclinical hypothyroid and carotid artery intima media thickness. Four studies met the criteria and were included in the analysis.
Results: The four studies analyzed indicated that treatment with levothyroxine resulted in a significant decrease in CIMT in subjects with SCH. Correlations were found between CIMT and lipid profile variables. Additionally, age, LDL-C, systolic blood pressure, and anti-thyroglobulin antibodies were important predictors of CIMT variation.
Conclusion: Treatment of subclinical hypothyroidism with levothyroxine decreases carotid artery intima media thickness, which is an important measure of atherosclerosis and cardiovascular risk. Although debate still exists whether SCH should be screened for, and treated in clinical practice, considerations should be made based on the cardiovascular risk and history of the patient on an individual basis. Additional randomized controlled trials with a greater number of subjects are needed to further investigate the effects of SCH treatment on CIMT and other cardiovascular markers. Moreover, research is needed to further illuminate the long-term effects of SCH treatment and cardiovascular risks and events.
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