Some patients use liothyronine as thyroid replacement therapy as an alternative to L-thyroxine. Trials have examined the potential benefits, but there is little data looking at the relative safety of these two agents. All patients receiving thyroid replacement therapy between 1993 and 2014 in Tayside were included in a cohort study (n=34 355; 319 500 patient years of follow up). Overall 33 955 patients received only L-thyroxine, and 327 received liothyronine in combination with L-thyroxine and 73 on liothyronine alone (total=400). Using unique patient identification numbers, biochemical, prescribing, hospital admission, radioiodine and general registry office data were linked.
Patients initiating treatment with liothyronine were younger (48 vs 59 years P<0.001), but there was no gender difference (85% female vs 82%). They were more likely at baseline to have had thyroid cancer, have a history of previous hyperthyroidism and be treated with anti-psychotic or anti-depressant medication. They were less likely to have cardiovascular disease or be treated with a statin. During a mean follow up of 9.3 years (±5.6 years) proportional hazards ratios (HR) were reported after adjustment for age, gender, baseline TSH, number of thyroid prescriptions and history of thyroid cancer or hyperthyroidism. For patients taking liothyronine there was no increased risk of death (0.78; 95% confidence interval: 0.541.11), fractures (HR 0.79; 0.491.27), atrial fibrillation (HR 0.91; 0.471.75) or cardiovascular disease (HR 0.90; 0.421.92). There was an increased risk of mental health disorders (HR 3.27; 1.0210.52) for patients taking liothyronine alone, but not for those taking a combination therapy. There was an increased incident use of anti-psychotic medication (HR 2.26; 1.643.11).
No increased risk of fractures or atrial fibrillation in patients taking liothyronine compared to L-thyroxine was demonstrated. There was an increased risk of mental health disorders if liothyronine was used alone.