How should a pregnant woman with a three‑month history of palpitations, hemoglobin 10.2 g/dL, and hypothyroidism treated with 100 µg levothyroxine (Thyronorm) and recent normal thyroid function tests be evaluated and managed?

Approach to Palpitations in Pregnancy with Controlled Hypothyroidism and Mild Anemia

The palpitations in this pregnant woman are most likely physiological or related to her mild anemia (Hb 10.2 g/dL) rather than thyroid dysfunction, since her thyroid function tests are normal on adequate levothyroxine replacement; focus on excluding cardiac causes, optimizing hemoglobin, and reassuring the patient while continuing thyroid monitoring.

Immediate Assessment Priorities

1. Cardiac Evaluation

• Obtain ECG to rule out arrhythmias (atrial fibrillation, supraventricular tachycardia, or premature contractions)
• Check vital signs including resting heart rate and blood pressure
• Assess for cardiac symptoms: chest pain, dyspnea at rest, syncope, or presyncope
• If ECG abnormal or concerning symptoms present, consider echocardiography to exclude structural heart disease or peripartum cardiomyopathy

2. Anemia Management

The hemoglobin of 10.2 g/dL represents mild anemia in pregnancy (WHO cutoff <11 g/dL in first/third trimester, <10.5 g/dL in second trimester). Anemia is a known cause of palpitations and can exacerbate symptoms 1.

• Check complete blood count with indices (MCV, MCH, RDW)
• Obtain iron studies: serum ferritin, serum iron, TIBC, transferrin saturation
• Check vitamin B12 and folate levels (hypothyroidism can coexist with B12 deficiency causing hematological abnormalities) 2
• Start oral iron supplementation (60-120 mg elemental iron daily) empirically while awaiting results
• Recheck hemoglobin in 4 weeks

3. Thyroid Management Verification

Although TFTs were normal 3 weeks ago, pregnancy increases levothyroxine requirements throughout gestation 3.

Key actions:

• Repeat TSH and free T4 now to confirm continued euthyroid status 3
• TSH should be maintained in trimester-specific ranges (first trimester: 0.1-2.5 mIU/L) 4
• Monitor TSH every 4 weeks during pregnancy as per FDA labeling 3
• If TSH elevated above trimester-specific range, increase levothyroxine by 12.5-25 mcg/day 3

Important caveat: Levothyroxine requirements increase as early as 5 weeks gestation and plateau by week 16, with an average 30-50% dose increase needed 5, 6. Since she's on 100 mcg and already 3+ months pregnant, verify her dose was appropriately increased early in pregnancy.

Physiological Palpitations in Pregnancy

Normal pregnancy causes:

• Increased cardiac output (30-50% above baseline)
• Increased heart rate (10-20 bpm above baseline)
• Increased stroke volume
• Enhanced awareness of normal heartbeat due to increased blood volume

These are the most likely explanation if cardiac workup is normal and anemia is mild.

Management Algorithm

Step 1: ECG + vital signs today

• If abnormal → cardiology referral
• If normal → proceed to Step 2

Step 2: Address anemia

• Start iron supplementation immediately
• Check iron studies, B12, folate
• Recheck CBC in 4 weeks
• Target hemoglobin >11 g/dL

Step 3: Verify thyroid control

• Repeat TSH and free T4 now
• If TSH >2.5 mIU/L (first trimester) or above trimester-specific range → increase levothyroxine by 12.5-25 mcg 3
• Continue monthly TSH monitoring throughout pregnancy 3

Step 4: If all workup normal

• Reassure patient about physiological nature
• Avoid triggers: caffeine, stress, dehydration
• Encourage adequate hydration and rest
• Follow-up in 2-4 weeks to reassess symptoms after anemia treatment

Critical Pitfalls to Avoid

1. Do not assume thyroid is adequately controlled based on a single normal test 3 weeks ago; pregnancy is dynamic and requirements change 7, 5

2. Do not overlook anemia as a cause of palpitations; even mild anemia (Hb 10.2) can cause significant symptoms and is associated with hypothyroidism 1

3. Avoid overtreatment with levothyroxine, as TSH <0.10 mIU/L is associated with preterm delivery (adjusted OR 2.14) 7

4. Do not dismiss palpitations as "just pregnancy" without excluding arrhythmias via ECG

5. Remember that normocytic normochromic anemia is common in thyroid dysfunction 1, so investigate beyond just iron deficiency

Monitoring Through Pregnancy

• TSH and free T4: Every 4 weeks until stable, then each trimester minimum 3
• Hemoglobin: Every 4 weeks until normalized, then each trimester
• Clinical assessment: Ongoing evaluation of palpitation frequency and severity
• Postpartum: Reduce levothyroxine to pre-pregnancy dose immediately after delivery 3