No Antithyroid Treatment Required
A 28-week pregnant woman with suppressed TSH, low free T3, and normal free T4 does not require antithyroid medication—this pattern represents normal pregnancy physiology, not pathological hyperthyroidism. 1
Understanding Normal Pregnancy Thyroid Physiology
During normal pregnancy, thyroid function tests undergo predictable physiological changes that can mimic thyroid disease:
- TSH naturally decreases during pregnancy, particularly in the first trimester due to hCG cross-reactivity with the TSH receptor, and may remain mildly suppressed throughout gestation 1
- Free T4 levels decline progressively as pregnancy advances, with values in the third trimester often falling to the lower end of the non-pregnant reference range or even slightly below 1
- Free T3 similarly decreases during the second and third trimesters, while total T3 remains elevated due to increased thyroid-binding globulin 1
- These changes represent normal adaptation, not thyroid dysfunction, and occur in healthy pregnant women with adequate iodine intake 2
Why This Patient Does Not Have Hyperthyroidism
The combination of low TSH, low free T3, and normal free T4 at 28 weeks gestation is inconsistent with true hyperthyroidism:
- True hyperthyroidism (Graves' disease or toxic nodular goiter) would present with elevated free T4 and/or free T3, not low free T3 with normal free T4 3
- Gestational transient thyrotoxicosis (from hCG excess) occurs in the first trimester and resolves by mid-pregnancy, not at 28 weeks 3
- The normal free T4 at 28 weeks excludes clinically significant thyrotoxicosis requiring treatment 3, 4
Clinical Decision Algorithm
Do NOT initiate antithyroid medication when:
- TSH is suppressed but free T4 is normal or low-normal 3
- Free T3 is low rather than elevated 1
- Patient is in second or third trimester without symptoms of thyrotoxicosis 3
DO initiate antithyroid medication only when:
- Free T4 (or free thyroxine index) is elevated above the pregnancy-specific reference range 3
- Clinical signs of hyperthyroidism are present: tachycardia >100 bpm at rest, tremor, heat intolerance, weight loss despite adequate intake 3
- TSH is suppressed and free T3 is markedly elevated 3
Monitoring Recommendations
For this patient with normal free T4 at 28 weeks:
- Recheck TSH and free T4 in 4 weeks to confirm stability 3
- Assess for clinical hyperthyroid symptoms: resting heart rate, tremor, excessive sweating, weight loss 3
- No treatment is indicated unless free T4 becomes elevated or symptoms develop 3
- Inform the newborn's physician at delivery about maternal thyroid status for neonatal monitoring 3
Critical Pitfalls to Avoid
- Never treat based on TSH alone during pregnancy—TSH suppression is physiological in 10-15% of normal pregnancies 1, 4
- Never use radioactive iodine during pregnancy, as it causes fetal thyroid ablation 3
- Do not target mid-normal free T4 levels if treatment were needed—the goal in true hyperthyroidism is high-normal free T4 to avoid fetal hypothyroidism 3
- Recognize assay variability: free hormone immunoassays are notoriously unreliable during pregnancy due to altered binding proteins; clinical correlation is essential 4, 5
When to Reconsider the Diagnosis
Reassess for true hyperthyroidism if:
- Free T4 rises above the pregnancy-specific upper limit on repeat testing 3
- Persistent tachycardia (>100 bpm at rest) develops 3
- Unintentional weight loss occurs despite adequate caloric intake 3
- Thyroid storm features emerge: fever, altered mental status, heart failure (medical emergency requiring immediate treatment) 3