When should thyroid hormone replacement be started in an adult with a TSH of 6.7 mIU/L?

When to Start Thyroid Medication for TSH 6.7 mIU/L

For a TSH of 6.7 mIU/L, confirm the elevation with repeat testing in 3–6 weeks before initiating treatment, because 30–60% of mildly elevated TSH values normalize spontaneously. 1, 2

Initial Confirmation Steps

• Repeat TSH and measure free T4 after 3–6 weeks to verify persistent elevation and distinguish subclinical hypothyroidism (normal free T4) from overt hypothyroidism (low free T4). 1, 2
• A TSH of 6.7 mIU/L falls in the "gray zone" between 4.5–10 mIU/L where treatment decisions require individualized assessment rather than automatic initiation. 1
• Measure anti-TPO antibodies to identify autoimmune thyroiditis, which predicts higher progression risk (4.3% per year vs 2.6% in antibody-negative patients) and may influence treatment decisions. 1

Treatment Algorithm Based on Confirmed TSH Levels

TSH 4.5–10 mIU/L with Normal Free T4 (Subclinical Hypothyroidism)

Routine levothyroxine treatment is NOT recommended for asymptomatic patients, as randomized controlled trials found no symptomatic improvement with therapy. 1

Consider treatment in specific situations:

• Symptomatic patients with fatigue, weight gain, cold intolerance, or constipation may benefit from a 3–4 month trial of levothyroxine with clear evaluation of benefit. 1
• Pregnant women or those planning pregnancy should be treated for any TSH elevation, targeting TSH <2.5 mIU/L in the first trimester, because subclinical hypothyroidism is associated with preeclampsia, low birth weight, and potential neurodevelopmental effects. 1, 3
• Positive anti-TPO antibodies identify patients with 4.3% annual progression risk who may warrant earlier treatment. 1
• Goiter or infertility may justify treatment consideration. 1

If observation is chosen, monitor TSH and free T4 every 6–12 months. 1, 2

TSH >10 mIU/L (Regardless of Symptoms)

Initiate levothyroxine therapy immediately, as this threshold carries approximately 5% annual risk of progression to overt hypothyroidism and is associated with cardiac dysfunction (delayed relaxation, abnormal cardiac output) and adverse lipid profiles (elevated LDL cholesterol). 1

Levothyroxine Dosing Guidelines

For patients <70 years without cardiac disease:

• Start with full replacement dose of approximately 1.6 mcg/kg/day. 1

For patients >70 years OR with cardiac disease/multiple comorbidities:

• Start with lower dose of 25–50 mcg/day and titrate gradually by 12.5–25 mcg every 6–8 weeks to avoid unmasking cardiac ischemia or precipitating arrhythmias. 1

Monitoring Protocol

• During dose titration: Monitor TSH every 6–8 weeks after any dose adjustment until TSH reaches target range of 0.5–4.5 mIU/L. 1
• Once stable: Repeat testing every 6–12 months or if symptoms change. 1
• Free T4 can help interpret ongoing abnormal TSH levels during therapy, as TSH may take longer to normalize. 1

Critical Safety Considerations

Before initiating levothyroxine:

• Rule out adrenal insufficiency, especially in suspected central hypothyroidism, by checking morning cortisol and ACTH, because starting thyroid hormone before corticosteroids can precipitate life-threatening adrenal crisis. 1
• If adrenal insufficiency is present, start hydrocortisone at least 1 week before levothyroxine. 1

Common Pitfalls to Avoid

• Do not treat based on a single elevated TSH value without confirmation, as transient elevations are common during acute illness, recovery from thyroiditis, or after iodine exposure. 1, 2
• Avoid overtreatment, which occurs in 14–21% of treated patients and increases risk for atrial fibrillation (3–5 fold), osteoporosis, fractures, and cardiovascular mortality, especially in elderly patients. 1
• Approximately 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH (<0.1 mIU/L), creating serious complication risks. 1
• Do not overlook age-related TSH changes: In patients >80 years, TSH values up to 7.5 mIU/L may represent normal aging rather than pathology, and the upper reference limit shifts upward with age. 2, 4

Special Population Considerations

Elderly patients (>70 years):

• Use conservative approach with lower starting doses (25–50 mcg/day) if treatment becomes necessary. 2
• Monitor closely for cardiac complications. 2
• Consider that 12% of persons aged ≥80 years with no thyroid disease have TSH >4.5 mIU/L, making standard reference ranges potentially inappropriate. 2

Patients on immune checkpoint inhibitors:

• Consider treatment even for subclinical hypothyroidism if fatigue or other symptoms are present, as thyroid dysfunction occurs in 6–9% with anti-PD-1/PD-L1 therapy. 1
• Continue immunotherapy in most cases, as thyroid dysfunction rarely requires treatment interruption. 1