Why is TSH (Thyroid Stimulating Hormone) level checked first in pregnant patients with suspected hyperthyroidism, rather than T4 (Thyroxine) or T3 (Triiodothyronine)?

TSH as First-Line Test for Suspected Hyperthyroidism in Pregnancy

TSH is checked first in pregnant patients with suspected hyperthyroidism because it is the most sensitive initial screening test for thyroid dysfunction, as recommended by the American Academy of Family Physicians, with abnormal values prompting further testing of Free T4 or Free T4 Index. 1

Rationale for TSH as Initial Test

• TSH is the most sensitive indicator of thyroid dysfunction and responds to even subtle changes in thyroid hormone levels through the negative feedback loop
• TSH levels decrease in the first trimester due to hCG's thyroid-stimulating effect, then increase in second and third trimesters 2
• A suppressed TSH is often the earliest laboratory abnormality in hyperthyroidism, appearing before T3 or T4 elevations become evident

Pregnancy-Specific Considerations

1. Physiologic changes affecting thyroid tests:

   • Pregnancy causes significant changes in thyroid function:
     • 50% increase in thyroxine-binding globulin (TBG) 2
     • 30-100% increase in total T3 and T4 levels 3
     • Increased serum thyroglobulin
     • Increased renal iodide clearance

2. hCG cross-reactivity:

   • Human chorionic gonadotropin (hCG) has mild thyroid-stimulating activity 3
   • Can cause transient physiologic suppression of TSH in first trimester
   • May lead to "gestational transient thyrotoxicosis," especially with hyperemesis gravidarum 4

Testing Algorithm for Suspected Hyperthyroidism in Pregnancy

1. Step 1: Check TSH level

   • If normal or elevated → unlikely to be hyperthyroidism
   • If suppressed → proceed to step 2

2. Step 2: Measure Free T4 (FT4) or Free T4 Index (FTI)

   • If elevated with suppressed TSH → confirms hyperthyroidism
   • If normal with suppressed TSH → possible T3 toxicosis, check Free T3

3. Step 3: Consider thyroid antibody testing

   • TSH receptor antibodies (TRAb) to identify Graves' disease
   • Important to differentiate from gestational transient thyrotoxicosis

Clinical Implications

• Untreated hyperthyroidism in pregnancy can lead to:

  • Heart failure
  • Spontaneous abortion
  • Preterm birth
  • Stillbirth 1

• Proper diagnosis enables appropriate treatment:

  • Propylthiouracil preferred in first trimester
  • Methimazole preferred in second and third trimesters 1
  • Goal: maintain Free T4 in high-normal range using lowest possible medication dose

Common Pitfalls to Avoid

• Pitfall #1: Relying on total T3/T4 levels without considering pregnancy-related TBG increases

  • Solution: Always use Free T4 or Free T4 Index, not total hormone levels 3

• Pitfall #2: Misinterpreting first-trimester TSH suppression as pathologic

  • Solution: Consider normal physiologic suppression due to hCG effects

• Pitfall #3: Failing to recognize gestational transient thyrotoxicosis

  • Solution: Consider this diagnosis in women with hyperemesis gravidarum and suppressed TSH 4

• Pitfall #4: Missing maternal Graves' disease that could affect the fetus

  • Solution: Test for TSH receptor antibodies when hyperthyroidism is confirmed

Monitoring Recommendations

• TSH and Free T4 measurements should be taken as soon as pregnancy is confirmed
• Continue monitoring at minimum during each trimester
• More frequent monitoring (every 4-6 weeks) until TSH levels stabilize 1
• For hyperthyroidism treatment, monitor Free T4 every 2-4 weeks 1