What is the appropriate management for a 35-year-old underweight woman with hypothyroidism, characterized by an elevated TSH (Thyroid-Stimulating Hormone) level and low T3 (Triiodothyronine) and T4 (Thyroxine) levels?

Immediate Levothyroxine Therapy Required for Overt Primary Hypothyroidism

This 35-year-old woman has severe overt primary hypothyroidism requiring immediate levothyroxine treatment at a starting dose of approximately 70 mcg daily (1.6 mcg/kg × 45 kg), with dose titration every 6-8 weeks until TSH normalizes to 0.5-4.5 mIU/L. 1, 2

Diagnosis Confirmation

Your patient's laboratory values definitively establish overt primary hypothyroidism:

• TSH 21 mIU/L (markedly elevated, normal 0.45-4.5 mIU/L) 1
• T4 7 µg/dL (low, assuming normal range ~5-12 µg/dL) 2
• T3 1.1 ng/mL (low-normal to low) 2

This combination of elevated TSH with low thyroid hormones indicates the thyroid gland itself is failing (primary hypothyroidism), not a pituitary problem. 1, 2 The TSH >10 mIU/L threshold alone mandates treatment regardless of symptoms. 1, 3

Initial Treatment Protocol

Starting Dose Calculation

For this 45 kg, 35-year-old woman without cardiac disease, start levothyroxine at 70 mcg daily (1.6 mcg/kg/day × 45 kg = 72 mcg, round to available 75 mcg tablet). 1, 4 Since she is under 70 years old and presumably without significant cardiac comorbidities, full replacement dosing is appropriate rather than conservative titration. 1, 3

Critical Safety Consideration

Before initiating levothyroxine, you must rule out concurrent adrenal insufficiency, as starting thyroid hormone before corticosteroids can precipitate life-threatening adrenal crisis. 1 Check morning cortisol and ACTH if there are any signs of adrenal insufficiency (hypotension, hyponatremia, hyperpigmentation). 1 If adrenal insufficiency is present, start physiologic dose steroids 1 week prior to thyroid hormone replacement. 1

Administration Instructions

• Take levothyroxine on an empty stomach, 30-60 minutes before breakfast 1, 4
• Separate from iron, calcium supplements, or antacids by at least 4 hours 5
• Avoid proton pump inhibitors if possible, as they reduce absorption 5

Monitoring and Dose Adjustment

Initial Monitoring Schedule

Recheck TSH and free T4 in 6-8 weeks after starting treatment, as this represents the time needed to reach steady state. 1, 2 Do not adjust the dose more frequently, as premature changes lead to inappropriate dosing. 1

Dose Titration Strategy

• Adjust by 12.5-25 mcg increments based on TSH response 1
• Target TSH: 0.5-4.5 mIU/L with normal free T4 1, 3
• Continue checking TSH every 6-8 weeks until stable 1
• Once stable, monitor TSH annually or if symptoms change 1, 2

Expected Clinical Improvements

With adequate treatment, expect:

• Symptom resolution within 6-8 weeks: fatigue, cold intolerance, weight changes, constipation 2, 4
• Cardiovascular function improvement: reduced heart failure risk, improved cardiac output 1, 2
• Metabolic normalization: improved insulin sensitivity if diabetic, reduced LDL cholesterol 1, 3
• Reproductive health restoration: normalized menstrual cycles, improved fertility 2

Critical Pitfalls to Avoid

Undertreatment Risks

• Persistent hypothyroid symptoms affecting quality of life 1
• Cardiovascular dysfunction including heart failure 1, 2
• Adverse lipid profiles 1
• Infertility and pregnancy complications 2, 4
• Risk of progression to myxedema coma (30% mortality) 2

Overtreatment Risks

Approximately 25% of patients on levothyroxine are unintentionally maintained on excessive doses, leading to: 1

• Atrial fibrillation (3-5 fold increased risk with TSH <0.1 mIU/L) 1
• Osteoporosis and fractures, especially in postmenopausal women 1, 6
• Cardiovascular mortality 1
• Cardiac arrhythmias 1

Common Errors

• Do not treat based on single TSH measurement without confirmation - however, with TSH 21 mIU/L and low T4, this is clearly overt hypothyroidism requiring immediate treatment 1, 6
• Do not assume hypothyroidism is always permanent - consider transient thyroiditis, though with TSH this elevated, permanent hypothyroidism is most likely 1, 6
• Do not start thyroid hormone before ruling out adrenal insufficiency in suspected central hypothyroidism 1

Special Considerations for This Patient

Low Body Weight (45 kg)

Her underweight status may reflect:

• Severe hypothyroidism causing decreased appetite 2
• Possible concurrent malabsorption affecting both weight and levothyroxine absorption 1
• Need for careful monitoring as weight normalizes with treatment 2

Reproductive Age Considerations

If she plans pregnancy, optimize thyroid function immediately, targeting TSH <2.5 mIU/L before conception. 1, 4 Untreated hypothyroidism increases risks of:

• Miscarriage 2, 4
• Preeclampsia 1, 4
• Low birth weight 1, 4
• Permanent neurodevelopmental deficits in offspring 1, 4

If she becomes pregnant, immediately increase levothyroxine by 25-50% (take one extra dose twice weekly), then check TSH monthly. 1, 4

Long-Term Management

Once TSH normalizes:

• Annual TSH monitoring for stable patients 1, 2
• Lifelong treatment is typically required for primary hypothyroidism 2, 3
• Ensure adequate calcium (1200 mg/day) and vitamin D (1000 units/day) to prevent bone loss if TSH becomes suppressed 1
• Monitor for other autoimmune conditions if Hashimoto thyroiditis is confirmed with anti-TPO antibodies 1, 3