A Cochrane review included 12 studies with a total of 350 subjects. Eleven trials investigated levothyroxine replacement with placebo, one study compared levothyroxine replacement with no treatment. No studies were identified that assessed (cardiovascular) mortality or morbidity. Seven studies evaluated symptoms, mood and quality of life with no statistically significant improvement. One study showed a statistically significant improvement in cognitive function. Six studies assessed serum lipids, there was a trend for reduction in some parameters following levothyroxine replacement. Some echocardiographic parameters improved after levothyroxine replacement therapy, like myocardial relaxation, as indicated by a significant prolongation of the isovolumic relaxation time as well as diastolic dysfunction. Only four studies reported adverse events with no statistically significant differences between groups.
Comment: The quality of evidence is downgraded by study quality (unclear allocation concealment).