Check TSH + Free T4
- TSH ↑ + Free T4 ↓ → Overt Hypothyroidism.
- TSH ↑ + Free T4 normal → Subclinical Hypothyroidism (SCH).
If Overt Hypothyroidism
- Start Levothyroxine:
- Typical starting dose: 1.6 mcg/kg/day (younger, healthy adults).
- Elderly or cardiac disease: start 25–50 mcg/day, titrate as tolerated.
- Recheck TSH in 6–8 weeks.
- Goal: normalize TSH (0.5–4.5), target closer to 2.5 if pregnant.
If Subclinical Hypothyroidism (TSH ↑, T4 normal)
Treat if:
- TSH ≥10 mIU/L.
- TSH 4.5–9.9 mIU/L with any of: hypothyroid symptoms, positive anti-TPO antibodies, goiter, pregnancy or planning pregnancy, age <65 with risk factors (CV disease, metabolic syndrome, infertility).
Otherwise: observe + recheck TSH in 6–12 months.
Monitoring
- Recheck TSH 6–8 weeks after dose changes, then every 6–12 months once stable.
Referral indications
- Persistent TSH elevation despite adequate dose & adherence.
- Suspected central hypothyroidism (low/normal TSH with low free T4).
- Goiter or thyroid nodules.
- Pregnancy with hypothyroidism (early/urgent).
- New/worsening symptoms despite treatment (early/urgent).
- Suspicious features (Urgent): rapidly enlarging goiter, hard/fixed mass, compressive symptoms, cervical lymphadenopathy, personal/family history of thyroid cancer, prior H&N radiation.
Important considerations (Levothyroxine dosing)
- Weight for calculation:
- BMI <30 → use actual body weight at ~1.6 mcg/kg/day.
- BMI ≥30 → use Ideal Body Weight (IBW):
- Men: 50 kg + 0.9 × (height cm − 152).
- Women: 45.5 kg + 0.9 × (height cm − 152).
- Online calculator: MDCalc IBW.
- Rounding doses: tablets available in increments of 25 mcg. Round to nearest available strength. Midway (e.g., 84 mcg between 75 and 100): younger/healthy → round up; elderly/cardiac → round down.