Subclinical Hypothyroidism: TSH 18 mIU/L with Normal T3/T4

What This Pattern Means

Most Likely Causes to Investigate

1. Hashimoto's thyroiditis (most common) - order anti-TPO antibodies (TPOAb) and anti-thyroglobulin antibodies
2. Iodine deficiency
3. Prior thyroid surgery or radioiodine treatment
4. Medications - lithium, amiodarone, checkpoint inhibitors, interferon
5. Thyroid infiltrative disease

Key Next Steps

• Repeat TSH + free T4 in 2-3 months to confirm the elevation is sustained, not transient (e.g., from recovery phase of thyroiditis or a sick-euthyroid state)
• Check anti-TPO antibodies - positivity significantly raises risk of progression to overt hypothyroidism and affects treatment decision

Management Decision Framework

• TSH confirmed elevated on repeat testing (after 2-3 months)
• TSH >10 mIU/L - your patient at 18 fits this threshold
• Patient is symptomatic (fatigue, cold intolerance, constipation, weight gain, bradycardia)
• TPOAb positive (high progression risk)
• Patient has elevated LDL or cardiovascular risk factors
• Woman wishing to conceive or pregnant (treat without waiting)

• Elderly (≥70 years) - evidence for benefit is weaker in older adults; risk of over-treatment (arrhythmia, osteoporosis) must be weighed
• Patient asymptomatic with no cardiovascular risk and TSH is newly discovered (wait for repeat confirmation first)

Levothyroxine Dosing

• Standard starting dose: 25-50 mcg/day (Harrison's)
• Elderly or known coronary artery disease: start at 12.5-25 mcg/day, titrate every 2-3 months
• Goal: normalize TSH (typically 0.5-2.5 mIU/L)
• Recheck TSH 4-6 weeks after any dose change
• Once stable, annual TSH monitoring is sufficient

Special Populations

• Pregnant / wanting to conceive: Treat immediately without waiting for confirmatory repeat. Target TSH <2.5 mIU/L. Monitor every 4 weeks in first half of pregnancy.
• Elderly: Balance benefit vs. risk of over-replacement (atrial fibrillation, bone loss).

Summary for Your Patient

1. Confirm with a repeat TSH + free T4 in 6-8 weeks (unless pregnant)
2. Check TPOAb to identify Hashimoto's and assess progression risk
3. Start LT4 25-50 mcg/day - at TSH 18, treatment is indicated per all major guidelines
4. Recheck TSH in 4-6 weeks and titrate to target

Investigations

Tier 1 - Essential (Do Now)

Tier 2 - Targeted by Clinical Context

Treatment

Step 1 - Confirm Before Treating (Unless Pregnant)

Exception: If the patient is pregnant or planning conception - start LT4 immediately without waiting.

Step 2 - Start Levothyroxine (LT4)

• Take on an empty stomach, at least 30 minutes before breakfast
• Separate from iron supplements, calcium, PPI, cholestyramine, sucralfate - these all impair LT4 absorption (Harrison's)
• If patient misses a dose - can take two doses the next day (T4 half-life is 7 days)

Step 3 - TSH Target and Monitoring

Step 4 - Escalation and Dose Adjustment

• If TSH remains high despite adequate dose: check adherence first (the most common cause)
• Poor adherence pattern: normal/high free T4 + elevated TSH (patient takes pills before the test but not regularly - T4 normalizes quickly but TSH stays elevated for weeks)
• If truly malabsorbing: investigate celiac disease, H. pylori gastritis, small bowel disease
• Full replacement dose for most adults: 1.6 mcg/kg/day

What NOT to Do

• Do not use T3 (liothyronine) alone - short half-life causes fluctuating T3 levels
• Do not use desiccated thyroid extract - T3:T4 ratio is non-physiologic
• Do not start LT4 if adrenal insufficiency is suspected without first treating with hydrocortisone (risk of adrenal crisis)
• Do not over-treat - suppressed TSH increases risk of atrial fibrillation and osteoporosis (especially in elderly and post-menopausal women)

Monitoring for Complications of Under/Over-Treatment

Summary at a Glance

TSH 18 (confirmed) + normal T3/T4
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Check: TPOAb, anti-Tg, lipids, FBC, U&E
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Start LT4 50 mcg/day (young adult)
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Recheck TSH at 6-8 weeks
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Titrate up by 25 mcg increments
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Target TSH 0.5–2.5 mIU/L
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Annual TSH once stable