When should treatment for hypothyroidism be initiated versus repeating the Thyroid-Stimulating Hormone (TSH) test?

When to Start Treatment vs. Repeat TSH Testing

Start levothyroxine immediately for TSH persistently >10 mIU/L or for symptomatic patients at any TSH elevation; otherwise, repeat TSH in 3-6 weeks before initiating treatment, as 30-60% of elevated TSH levels normalize spontaneously. 1

Initial Diagnostic Approach

Before making any treatment decision, confirm the elevated TSH with repeat testing after 3-6 weeks, along with free T4 measurement. 1 This confirmation step is critical because:

• 30-60% of initially elevated TSH levels normalize on repeat testing 1, 2
• A single elevated TSH may represent transient thyroiditis in recovery phase 1
• Measuring both TSH and free T4 distinguishes subclinical hypothyroidism (normal free T4) from overt hypothyroidism (low free T4) 1

Exception to the repeat testing rule: If TSH is markedly elevated (>60 mIU/L) or the patient has severe symptoms, confirm with free T4 but initiate treatment without waiting for repeat TSH. 3

Treatment Thresholds Based on TSH Level

TSH >10 mIU/L: Treat Immediately

Initiate levothyroxine therapy regardless of symptoms when TSH is persistently >10 mIU/L. 1 The rationale includes:

• Approximately 5% annual risk of progression to overt hypothyroidism 1, 4
• Higher risk justifies treatment to prevent complications 1
• Evidence supports treatment at this threshold even in asymptomatic patients 1, 5

TSH 4.5-10 mIU/L: Selective Treatment

Repeat TSH in 2-3 months and monitor without treatment for most asymptomatic patients in this range. 1, 5 Consider treatment only in specific circumstances:

• Symptomatic patients: Trial of levothyroxine for 3-4 months if fatigue, weight gain, cold intolerance, or constipation are present 1, 5
• Positive anti-TPO antibodies: Higher progression risk (4.3% vs 2.6% per year) warrants treatment consideration 1
• Pregnancy or planning pregnancy: Treat at any TSH elevation due to adverse pregnancy outcomes (preeclampsia, low birth weight, neurodevelopmental effects) 1, 6, 4
• Infertility or goiter: Treatment should be considered 4

Do not routinely treat asymptomatic patients with TSH 4.5-10 mIU/L, as double-blind randomized controlled trials show no improvement in symptoms or cognitive function with treatment in this range. 2

TSH <4.5 mIU/L: No Treatment

Normal TSH requires no intervention. 1

Special Population Considerations

Elderly Patients (>70-80 years)

Use a wait-and-see strategy for patients >80-85 years with TSH ≤10 mIU/L, generally avoiding treatment. 1, 5 The rationale:

• TSH naturally increases with aging regardless of thyroid disease 7
• Age-specific reference ranges show upper limit of normal TSH is 7.5 mIU/L for patients >80 years 2
• Treatment may be harmful in elderly patients with subclinical hypothyroidism 2

If treatment is necessary in elderly patients, start with 25-50 mcg/day rather than full replacement dose. 1, 4

Pregnant Patients

Treat immediately at any TSH elevation in pregnancy or when planning pregnancy. 1, 6, 4 Specific thresholds:

• TSH ≥10 mIU/L: Start 1.6 mcg/kg/day 6
• TSH <10 mIU/L: Start 1.0 mcg/kg/day 6
• Monitor TSH every 4 weeks during pregnancy 6
• Pre-existing hypothyroidism: Increase pre-pregnancy dose by 25-50% immediately upon pregnancy confirmation 6

Patients with Cardiac Disease

Start at lower doses (25-50 mcg/day) and titrate gradually in patients with coronary artery disease, atrial fibrillation, or multiple cardiac comorbidities. 1, 4 Consider more frequent monitoring (within 2 weeks rather than 6-8 weeks) after dose adjustments. 1, 3

Monitoring Schedule After Initiating Treatment

Once treatment is started:

• Recheck TSH and free T4 in 6-8 weeks after any dose change 1, 3, 6
• Target TSH 0.5-2.5 mIU/L (lower half of reference range) for most adults 5
• Once stable, monitor TSH every 6-12 months or when symptoms change 1, 3

Critical Pitfalls to Avoid

• Never treat based on a single elevated TSH without confirmation testing, as this leads to unnecessary lifelong treatment 1, 2
• Never start thyroid hormone before ruling out adrenal insufficiency in suspected central hypothyroidism, as this can precipitate adrenal crisis 1, 4
• Avoid overtreatment: 14-21% of treated patients develop iatrogenic hyperthyroidism, increasing risk for atrial fibrillation, osteoporosis, and fractures 1
• Don't ignore recent iodine exposure (CT contrast) which can transiently affect thyroid function tests 1
• Recognize that 25% of patients on levothyroxine are unintentionally maintained on excessive doses that fully suppress TSH 1