A 28‑week pregnant woman with low free T3, low TSH, and normal free T4—can she be cleared for hysteroscopy at 34 weeks?

Hysteroscopy at 34 Weeks with Low FT3, Low TSH, Normal FT4

This patient should NOT be cleared for hysteroscopy at 34 weeks until her thyroid function is fully evaluated and the underlying cause is determined, as this pattern may represent either a benign physiologic variant or a serious condition requiring immediate intervention.

Understanding the Thyroid Pattern

The combination of low FT3, low TSH, and normal FT4 at 28 weeks gestation is highly unusual and requires urgent clarification before any elective procedure 1:

• Normal pregnancy physiology shows elevated total T3 and T4 (due to increased thyroid-binding globulin), with TSH typically suppressed in the first trimester but normalizing thereafter 2, 3
• Free T4 and free T3 should remain within or slightly above the normal range during pregnancy, though they may decline slightly in the third trimester 2
• Low TSH with normal FT4 can occur physiologically in early pregnancy due to hCG stimulation, but at 28 weeks this pattern is abnormal 1
• Low FT3 alongside low TSH is particularly concerning and suggests either assay interference, non-thyroidal illness, or central hypothyroidism 1

Critical Differential Diagnoses

Familial Dysalbuminemic Hyperthyroxinemia (FDH)

• FDH causes artifactually elevated FT4 in one-step immunoassays due to abnormal albumin binding, but typically shows normal TSH and can present with elevated total T4 4
• This diagnosis is unlikely given the low FT3 and low TSH pattern, but assay interference must be excluded 4, 1

Central Hypothyroidism

• Low TSH with low FT3 raises concern for pituitary or hypothalamic dysfunction 5
• This is a medical emergency requiring immediate evaluation, as untreated maternal hypothyroidism increases risks of preeclampsia, low birth weight, and permanent neurodevelopmental deficits in the child 6
• Before any levothyroxine is started, adrenal insufficiency must be excluded by measuring morning cortisol and ACTH, as thyroid hormone replacement can precipitate life-threatening adrenal crisis 5, 6

Assay Interference

• Immunoassay methods for free T4 and free T3 are subject to interference from endogenous antibodies, medications, and pregnancy-related factors 1
• Repeat testing with a different assay method (e.g., equilibrium dialysis or tandem mass spectrometry) is essential to confirm the results 1

Non-Thyroidal Illness Syndrome

• Acute or chronic illness can suppress TSH and lower T3 levels while T4 remains normal 5
• However, this patient is at 28 weeks gestation and presumably not acutely ill, making this less likely unless there is an underlying medical condition 5

Immediate Diagnostic Workup Required

Before clearing for hysteroscopy, the following must be completed:

1. Repeat thyroid function tests with TSH, free T4, and free T3 using a different assay method to exclude interference 1
2. Measure total T3 and total T4 to assess for abnormal protein binding (e.g., FDH) 4, 1
3. Obtain morning cortisol and ACTH to exclude adrenal insufficiency, especially if central hypothyroidism is suspected 5, 6
4. Check anti-TPO antibodies to assess for autoimmune thyroid disease 6, 1
5. Review recent illness, medications, and iodine exposure (e.g., contrast agents) that could transiently affect thyroid function 5

Risks of Proceeding Without Evaluation

Maternal Risks

• Untreated hypothyroidism (if present) increases the risk of preeclampsia, which could complicate both the hysteroscopy and the remainder of the pregnancy 6
• Undiagnosed adrenal insufficiency could lead to adrenal crisis during the stress of surgery 5, 6
• Cardiac dysfunction from hypothyroidism (delayed relaxation, abnormal cardiac output) could increase anesthetic risk 5

Fetal Risks

• Inadequate maternal thyroid hormone in the third trimester can impair fetal neurodevelopment, as the fetus remains partially dependent on maternal T4 until delivery 6
• Untreated maternal hypothyroidism is associated with low birth weight and potential cognitive impairment in offspring 6

Management Algorithm

If Central Hypothyroidism is Confirmed

• Do NOT start levothyroxine until adrenal insufficiency is excluded 5, 6
• If adrenal insufficiency is present, initiate hydrocortisone (20 mg morning, 10 mg afternoon) for at least one week before starting levothyroxine 5
• Target TSH within trimester-specific reference ranges (ideally <2.5 mIU/L in pregnancy) 6
• Postpone hysteroscopy until thyroid function is stabilized and adrenal status is clarified 5, 6

If Assay Interference or FDH is Confirmed

• No treatment is required for FDH, as it is a benign condition 4
• Hysteroscopy can proceed at 34 weeks if maternal and fetal status are otherwise stable 4

If Non-Thyroidal Illness is Identified

• Address the underlying illness before proceeding with elective surgery 5
• Recheck thyroid function after resolution of the acute condition 5

Timing Considerations for Hysteroscopy

• Elective hysteroscopy at 34 weeks is unusual and should be reserved for urgent indications (e.g., retained products of conception, severe bleeding) 6
• If the procedure is truly elective, postpone until after delivery to avoid unnecessary fetal and maternal risk 6
• If the procedure is urgent, stabilize thyroid and adrenal function first unless the clinical situation is life-threatening 5, 6

Common Pitfalls to Avoid

• Do not assume normal thyroid function based on a single normal FT4 value when TSH and FT3 are both low 1
• Do not start levothyroxine empirically without excluding adrenal insufficiency, as this can precipitate adrenal crisis 5, 6
• Do not dismiss the low FT3 as a laboratory error without repeat testing using a different method 1
• Do not proceed with elective surgery until the thyroid abnormality is fully characterized and treated if necessary 5, 6