Is a TSH level of 0.05 possible with a baseline TSH of 3-4 and taking 75mcg of levothyroxine once a week?

Is TSH 0.05 Possible with Once-Weekly 75mcg Levothyroxine?

No, achieving a TSH of 0.05 from a baseline of 3-4 with only 75mcg levothyroxine taken once weekly is pharmacologically implausible—you almost certainly took it more frequently than you remember. 1

Pharmacokinetic Evidence Against Once-Weekly Dosing

Levothyroxine has a 7-day half-life, requiring 6-8 weeks to reach steady-state concentrations. 1 This long half-life means:

• Once-weekly dosing of 75mcg would provide an average daily dose of only ~10.7mcg per day 1
• The typical replacement dose for hypothyroidism is 1.6mcg/kg/day, which for most adults ranges from 100-150mcg daily 2, 3
• Your once-weekly regimen would deliver less than 10% of a standard replacement dose 1, 3

Why Your TSH Dropped So Dramatically

To suppress TSH from 3-4 down to 0.05 requires substantial thyroid hormone exposure over several weeks. 2, 1 The most likely explanations:

• You took levothyroxine daily or near-daily: A TSH of 0.05 indicates iatrogenic subclinical hyperthyroidism, which occurs in 14-21% of patients on levothyroxine therapy 4, 2
• The dose was likely 75mcg daily for several weeks: This would provide adequate hormone exposure to fully suppress TSH below 0.1 mIU/L 2, 1
• Memory error is common with medication adherence: Studies show that scheduling a levothyroxine absorption test can reveal that patients who believed they were non-adherent had actually been taking their medication 5

Mathematical Reality Check

The dose-response relationship makes once-weekly dosing impossible: 6

• To achieve TSH <1.0 mIU/L in athyreotic patients requires approximately 1.53-1.84 mcg/kg/day 6
• For a 70kg person, this equals 107-129mcg daily 6
• Your reported once-weekly 75mcg (10.7mcg/day equivalent) is only 8-10% of the required dose 6
• TSH suppression to 0.05 typically requires doses that fully replace or exceed physiologic needs 2, 6

Clinical Implications of TSH 0.05

This degree of TSH suppression carries significant health risks: 2

• Prolonged TSH <0.1 mIU/L increases risk for atrial fibrillation, especially in patients ≥45 years (5-fold increased risk) 2
• Increased risk of osteoporotic fractures, particularly in postmenopausal women 2
• Potential increased cardiovascular mortality 2
• Approximately 25% of patients on levothyroxine are unintentionally maintained on excessive doses 2

What Actually Happened

The most plausible scenario: 1, 5

1. You took 75mcg daily (or most days) for 6-8 weeks, allowing steady-state to be reached 1
2. This dose was excessive for your baseline mild hypothyroidism (TSH 3-4) 2
3. Your memory of "once weekly" dosing is inaccurate—medication adherence recall is notoriously unreliable 5
4. The pharmacokinetics simply do not support once-weekly dosing producing this result 1, 6

Recommended Action

If you currently have TSH 0.05 on any levothyroxine regimen: 2

• Reduce your dose by 25-50mcg immediately to prevent cardiovascular and bone complications 2
• Recheck TSH and free T4 in 6-8 weeks after dose adjustment 3
• Target TSH should be 0.5-4.5 mIU/L for primary hypothyroidism 2
• Never continue therapy that suppresses TSH below 0.1 mIU/L unless you have thyroid cancer requiring suppression 2