Combination Therapy with 137mcg Synthroid and 20mcg Liothyronine
Critical Assessment of Your Proposed Regimen
This combination dose is likely excessive and carries significant risk for iatrogenic hyperthyroidism, particularly with 20mcg of liothyronine, which substantially exceeds evidence-based recommendations for combination therapy. The standard approach when adding liothyronine involves reducing levothyroxine by 25mcg and adding only 2.5-7.5mcg of liothyronine once or twice daily 1. Your proposed 20mcg liothyronine dose is 2.5-8 times higher than recommended starting doses and could produce dangerous thyroid hormone elevations.
Why This Dose Is Problematic
Excessive Liothyronine Component
The 20mcg liothyronine dose far exceeds clinical guideline recommendations, which suggest starting with 2.5-7.5mcg when adding T3 to levothyroxine therapy 1.
Liothyronine has rapid onset of activity within hours, with maximum pharmacologic response occurring within 2-3 days and a biological half-life of approximately 2.5 days 2.
The rapid absorption (95% in 4 hours) and quick onset means excessive doses can rapidly produce symptomatic hyperthyroidism 2.
Studies following patients on combination therapy used mean daily T3 doses around 11mcg to normalize TSH, with your proposed 20mcg representing nearly double this amount 1.
Cardiovascular and Bone Risks
Overtreatment with thyroid hormone increases risk for atrial fibrillation, cardiac arrhythmias, left ventricular hypertrophy, osteoporosis, and fractures 3, 4.
Prolonged TSH suppression (which this dose would likely cause) carries substantial morbidity, especially in elderly patients, including accelerated bone loss in postmenopausal women 3.
Approximately 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, and your proposed regimen would almost certainly achieve this 3.
Supraphysiological thyroid hormone doses cause cardiovascular changes including shortened systolic time intervals, increased atrial premature beats, and possible left ventricular hypertrophy 4.
Evidence-Based Alternative Approach
Proper Combination Therapy Protocol
If you have confirmed overt hypothyroidism with persistent symptoms despite optimized levothyroxine monotherapy, reduce your Synthroid dose to 112mcg and add only 2.5-5mcg liothyronine twice daily 1, 5.
The reduction of 25mcg levothyroxine when adding liothyronine is critical because 25mcg of liothyronine is equivalent to approximately 100mcg of levothyroxine 2.
Target TSH should remain in the 0.3-2.0 mU/L range for 3-6 months before assessing therapeutic response 5.
Monitor TSH and free T4 every 6-8 weeks during dose titration 3, 6.
When Combination Therapy Is Appropriate
Combination therapy should only be considered for patients with confirmed overt hypothyroidism who remain symptomatic after adequate levothyroxine monotherapy with TSH optimized to 0.3-2.0 mU/L for 3-6 months 5.
Patients without clear biochemical evidence of overt hypothyroidism should first have a trial without thyroid hormone replacement 5.
The decision to start liothyronine should be a shared decision between patient and clinician, with clear evaluation of benefit 5.
Some evidence suggests patients carrying a polymorphism in the DIO2 gene may benefit more from combination therapy, though this requires confirmation 7.
Monitoring Requirements for Combination Therapy
Initial Monitoring Phase
Recheck TSH, free T4, and free T3 in 6-8 weeks after initiating combination therapy 3.
Assess for symptoms of hyperthyroidism including tachycardia, tremor, heat intolerance, weight loss, anxiety, or palpitations 3.
For patients with cardiac disease, atrial fibrillation, or serious medical conditions, consider more frequent monitoring within 2 weeks 8, 3.
Long-Term Surveillance
Once stable, monitor TSH every 6-12 months or sooner if symptoms change 3, 6.
An observational study following 400 patients on combination therapy for mean of 9 years showed no increased mortality or cardiovascular morbidity when properly dosed and monitored 1.
Development of suppressed TSH (<0.1 mU/L) indicates overtreatment requiring immediate dose reduction 8, 3.
Critical Pitfalls to Avoid
Never use combination therapy as first-line treatment—levothyroxine monotherapy remains the standard of care for newly diagnosed hypothyroidism 1, 9, 5.
Avoid excessive liothyronine doses that produce transient hypertriiodothyroninemia, though doses of 2.5-7.5mcg are unlikely to exceed reference range 1.
Do not adjust doses too frequently before reaching steady state—wait 6-8 weeks between adjustments 3.
Approximately 14-21% of patients on combination therapy develop subclinical hyperthyroidism, emphasizing the need for careful monitoring 3.
For patients with concurrent adrenal insufficiency, always initiate corticosteroids before starting or increasing thyroid hormone to prevent adrenal crisis 3, 6.