What are the implications and next steps for an adult patient with elevated TSH levels indicating hypothyroidism?

Management of TSH 5.9 mIU/L

Confirm the Diagnosis Before Any Treatment Decision

Do not initiate treatment based on a single TSH value of 5.9 mIU/L—repeat testing is mandatory. 1, 2

• Repeat TSH and measure free T4 after 3-6 weeks to confirm the elevation, as 30-60% of mildly elevated TSH levels normalize spontaneously 1, 2, 3
• If repeat TSH remains elevated with normal free T4, this confirms subclinical hypothyroidism 1, 4
• If repeat TSH normalizes, no treatment is needed—this likely represented transient thyroiditis or physiological variation 1

Treatment Decision Algorithm Based on Confirmed TSH Level

TSH 5.9 mIU/L Falls in the "Gray Zone" (4.5-10 mIU/L)

For TSH between 4.5-10 mIU/L with normal free T4, routine levothyroxine treatment is NOT recommended unless specific high-risk features are present. 1, 5, 3

Treat with levothyroxine if ANY of the following apply:

• Symptomatic patients with fatigue, weight gain, cold intolerance, or constipation—consider a 3-4 month trial with clear evaluation of benefit 1, 5
• Pregnant women or women planning pregnancy—treat immediately to prevent adverse pregnancy outcomes including preeclampsia, low birth weight, and neurodevelopmental effects 1, 5, 3
• Positive anti-TPO antibodies—these patients have 4.3% annual progression risk to overt hypothyroidism versus 2.6% in antibody-negative individuals 1, 5
• Presence of goiter 1, 5
• Infertility concerns 5

Do NOT treat if:

• Asymptomatic patient without the above risk factors 1, 3
• Age >85 years—treatment may be harmful in the very elderly 2
• Age >70-80 years with TSH <7.5 mIU/L—age-adjusted upper limit of normal TSH is 7.5 mIU/L in patients over 80 2, 3

If Treatment Is NOT Indicated

Monitor TSH and free T4 every 6-12 months to detect progression to overt hypothyroidism, which occurs at approximately 2-5% per year 1, 5

If Treatment IS Indicated

Initial Levothyroxine Dosing

For patients <70 years without cardiac disease:

• Start with full replacement dose of 1.6 mcg/kg/day 1, 6
• This rapidly normalizes thyroid function and prevents prolonged hypothyroid symptoms 1

For patients >70 years OR with cardiac disease:

• Start with 25-50 mcg/day and titrate gradually 1, 6, 5
• This prevents unmasking cardiac ischemia or precipitating arrhythmias 1, 5

Administration Instructions

• Take levothyroxine on an empty stomach, 30-60 minutes before breakfast with a full glass of water 1, 6
• Separate by at least 4 hours from iron, calcium supplements, or antacids 1, 6

Monitoring and Dose Adjustment

• Recheck TSH and free T4 in 6-8 weeks after starting treatment or any dose change 1, 6
• Target TSH: 0.5-2.5 mIU/L (lower half of normal range) 1, 3
• Adjust dose by 12.5-25 mcg increments every 6-8 weeks until TSH normalizes 1, 6
• Once stable, monitor TSH annually or sooner if symptoms change 1, 6

Critical Pitfalls to Avoid

• Never treat based on a single elevated TSH—62% may normalize spontaneously 2
• Never start thyroid hormone before ruling out adrenal insufficiency in suspected central hypothyroidism—this can precipitate adrenal crisis 1, 5
• Avoid overtreatment—14-21% of treated patients develop iatrogenic subclinical hyperthyroidism, increasing risk for atrial fibrillation, osteoporosis, and fractures 1
• Do not assume symptoms will improve with treatment in mild subclinical hypothyroidism (TSH <10 mIU/L)—randomized controlled trials show no symptom improvement in this range 2
• Recognize age-adjusted TSH reference ranges—TSH up to 7.5 mIU/L may be normal in patients >80 years 2, 3

Additional Diagnostic Testing to Consider

• Measure anti-TPO antibodies to identify autoimmune etiology and predict progression risk 1, 5, 3
• Thyroid ultrasound if goiter is suspected 7
• Review recent iodine exposure (CT contrast) which can transiently affect thyroid function 1