🦋 Subclinical Hypothyroidism Decision Simulator

Elevated TSH, Normal Free T4 — Subclinical Hypothyroidism Identified

Mildly high TSH with normal free T4, usually caught on routine labs.

• 4.5–10 mIU/L: TSH range (typical subclinical band)
• Normal: Free T4 (key distinguishing feature)
• ~4–10%: Adult prevalence (rises with age)
• Often none: Symptoms (mild fatigue possible)

What subclinical hypothyroidism means

TSH above normal, thyroid hormone still compensating adequately for now.

Why free T4 is the dividing line

Normal free T4 separates subclinical from overt, symptomatic hypothyroidism.

Where TSH Falls Shapes the Strength of the Treatment Case

Borderline TSH argues for watching; TSH above 10 argues for treating.

• Weak case: TSH <7 mIU/L (favors watchful monitoring)
• Mixed case: TSH 7–10 mIU/L (depends on other factors)
• Strong case: TSH >10 mIU/L (favors starting treatment)
• Rises with TSH: Progression risk (higher values, higher risk)

Reading the threshold tiers

Treatment guidelines lean harder toward therapy as TSH climbs higher.

Evidence strength by tier

Trial evidence for benefit is strongest only above roughly 10 mIU/L.

Age, Pregnancy, and Anti-TPO Antibodies Tilt the Scale

Younger age, pregnancy planning, and positive antibodies favor earlier treatment.

• Favors treat: Younger age (more benefit, fewer risks)
• Favors treat: Pregnancy / planning (protects fetal development)
• Favors treat: Anti-TPO positive (predicts progression to overt)
• Favors monitor: Older, no antibodies (lower progression likelihood)

Age as a modifier

Older patients tolerate untreated mild elevation better than younger ones.

Pregnancy changes the calculus

Fetal neurodevelopment raises the stakes during pregnancy or preconception.

Antibody positivity as a signal

Anti-TPO positivity predicts faster progression toward overt hypothyroidism.

Combining Factors Into a Single Treatment-Favoring Score

TSH weight, age, pregnancy, and antibody weight sum to tip the balance.

• 0–100 scale: Treat-favoring score (combined weighted factors)
• ≥ 50: Decision threshold (crossing point for treatment)
• Varies: Dominant factor (TSH usually weighted heaviest)
• Moderate: Confidence (genuinely contested territory)

How the weights combine

Each factor contributes a proportional share to the total score.

No single universal cutoff

Guidelines differ; clinicians weigh factors individually per patient.

Start Levothyroxine, or Monitor and Retest — The Outcome

The scale settles: begin treatment now, or recheck TSH in months.

• Treat or monitor: Recommendation (depends on accumulated score)
• 6–12 months: Retest interval (if monitoring is chosen)
• ~25–50 mcg/day: Starting dose (low-dose levothyroxine typical)
• Recheck TSH: Follow-up (6–8 weeks after starting)

When treatment is started

Levothyroxine begun low, titrated by repeat TSH testing over time.

When monitoring is chosen instead

Repeat labs in 6–12 months; reassess if symptoms or TSH change.