Tirosint Regret, Stopping, and Restarting: What Real Patients and Clinicians Say

What is Tirosint?

Tirosint is a soft gelatin capsule form of levothyroxine, not a tablet [1]. The capsule was approved in 2006, and Tirosint-SOL, an oral solution, was approved in 2016 [1]. Tirosint capsules are labeled for adults and for children 6 years and older who can swallow an intact capsule; the Synthroid tablet label extends down to neonates [1]. The detailed dosing, warning and adverse-reaction wording below comes from a levothyroxine sodium tablet label [2].

What are the risks of under- or over-treatment?

The levothyroxine label says levothyroxine has a narrow therapeutic index. It says undertreatment or overtreatment can affect growth and development, cardiovascular function, bone metabolism, reproductive function, cognitive function, gastrointestinal function, and glucose and lipid metabolism [2]. Levothyroxine has a half-life of 6 to 7 days in euthyroid patients and 9 to 10 days in hypothyroidism [2].

What does the label say about pregnancy?

The label says levothyroxine should not be discontinued during pregnancy and that hypothyroidism diagnosed during pregnancy should be promptly treated [2]. It also says pregnancy may increase levothyroxine requirements, TSH should be monitored and the dose adjusted, and the pre-pregnancy dose should be resumed immediately after delivery [2].

How is a restart dose judged?

The label says the starting dose depends on age, body weight, cardiovascular status and other medicines, and that dosing must be individualized based on clinical response and laboratory results [2]. For adults with hypothyroidism, the label gives a full replacement dose of 1.6 mcg/kg/day, with some patients needing a lower start. Dose titration is by 12.5 to 25 mcg increments every 4 to 6 weeks until the patient is euthyroid [2]. Adults at risk for atrial fibrillation, adults with underlying cardiac disease, and older adults have a lower starting dose, with titration every 6 to 8 weeks for cardiac patients [2]. The peak effect of a given dose may not be reached for 4 to 6 weeks [2]. In secondary or tertiary hypothyroidism, the label says to use free T4 rather than TSH [2].

Does the soft capsule absorb differently with a PPI?

A 2023 open-label study enrolled 47 thyroidectomized adults who were taking levothyroxine tablets and a proton pump inhibitor [3]. They switched to the same dose of Tirosint soft capsules for 12 weeks while continuing the PPI; 45 completed [3]. Mean TSH fell by 0.32 mIU/L at 6 weeks and 0.68 mIU/L at 12 weeks, with increased thyroid hormone levels [3]. The authors suggested the soft capsule may be less affected by PPIs [3]. The study had no control group and was sponsor-linked, and the Tirosint capsule label still lists PPIs among drugs that may reduce absorption [3].

What does the label say to watch for?

The label lists palpitations, tachycardia, arrhythmias, angina, heart failure, tremor, nervousness, insomnia, heat intolerance, sweating, weight loss, increased appetite, diarrhea, menstrual irregularities, and decreased bone mineral density as reactions mainly of hyperthyroidism from overdosage [2]. It says over-treatment may increase heart rate and cardiac contractility and may precipitate angina or arrhythmias, particularly in people with cardiovascular disease and older adults [2]. For cardiac symptoms that develop or worsen, the label says to reduce the dose or withhold for one week and restart at a lower dose [2].