Management of Normal TSH with Elevated FT4 in Third Trimester
Immediate Assessment Required
This presentation most likely represents normal pregnancy physiology in the third trimester and does not require treatment. However, you must first rule out several important conditions before concluding this is physiologic.
Diagnostic Approach
Confirm the Laboratory Findings
Verify that "normal TSH" truly falls within pregnancy-specific reference ranges, not standard non-pregnant ranges 1, 2. TSH reference intervals shift during pregnancy: first trimester 0.49-2.33 mIU/L, second trimester 0.51-3.44 mIU/L, and third trimester 0.58-4.31 mIU/L 2.
Recognize that TSH is not a reliable indicator of thyroid hormone status during pregnancy 3. In one large study of over 32,000 pregnancies, even when TSH exceeded 10 mIU/L in the first trimester, FT4 was below normal in only 12.8% of cases and FT3 in only 15.3% 3.
Measure FT3 in addition to FT4 and TSH to fully characterize thyroid status 4. During pregnancy, total T3 and T4 increase by 30-100% due to elevated thyroxine-binding globulin, but free hormone concentrations show only marginal changes in iodine-sufficient conditions 5.
Rule Out Pathologic Hyperthyroidism
Check TSH receptor antibodies (TRAb) to exclude Graves' disease, particularly if there are any clinical features of hyperthyroidism such as tachycardia, weight loss, tremor, or ophthalmopathy 1, 6.
Assess for symptoms of hyperthyroidism including persistent tachycardia (heart rate >100 bpm at rest), heat intolerance, excessive sweating, anxiety, or tremor 1. Beta-blockers like propranolol can be used temporarily if symptoms are present 1.
Examine for thyroid enlargement, bruit, or ophthalmopathy (proptosis, lid lag, periorbital edema), which would indicate Graves' disease requiring immediate endocrine referral 6.
Consider Gestational Transient Thyrotoxicosis
Evaluate for hyperemesis gravidarum, which can present with biochemical hyperthyroidism but rarely requires treatment 1. This condition is most common near the end of the first trimester when hCG levels peak 5, 4.
Understand that hCG has TSH-like effects and directly stimulates the maternal thyroid gland, particularly in the first trimester, which can transiently lower serum TSH 5, 4. This physiologic phenomenon explains why TSH may appear "normal" or even low-normal despite elevated FT4.
Management Algorithm
If Graves' Disease is Confirmed (Positive TRAb or Clinical Features)
Initiate antithyroid drug therapy with methimazole in the third trimester 1. Propylthiouracil (PTU) should only be used in the first trimester, with a switch to methimazole for the second and third trimesters 1.
Use the lowest effective dose to maintain FT4 in the high-normal range 1, 6. Monitor FT4 or free thyroxine index (FTI) every 2-4 weeks to guide dosage adjustments 1, 6.
Check TSH once per trimester as it is less reliable for monitoring during pregnancy 1.
Monitor for fetal complications including checking fetal heart rate and appropriate growth 1. Ultrasound screening for fetal goiter is not necessary unless problems are detected 1.
Warn about agranulocytosis risk and instruct the patient to report sore throat, fever, or signs of infection immediately 6.
If This is Physiologic (No TRAb, No Symptoms, Third Trimester)
No treatment is indicated 1, 5. The elevated FT4 with normal TSH likely represents normal pregnancy adaptation, particularly given the unreliability of TSH as a screening tool in pregnancy 3.
Recheck thyroid function postpartum at 6-8 weeks to ensure normalization 6. Some women develop postpartum thyroiditis, which can present with transient hyperthyroidism followed by hypothyroidism 6.
Monitor for postpartum thyroid dysfunction by measuring TSH and FT4 every 2-3 weeks for the first 3 months postpartum if the patient develops symptoms 6.
Critical Pitfalls to Avoid
Do not use non-pregnant reference ranges for TSH or free thyroid hormones, as this leads to misclassification 2. Pregnancy-specific reference intervals must be used 1, 2.
Do not assume TSH elevation or suppression accurately reflects thyroid hormone status in pregnancy 3. The thyrotropic effects of hCG, particularly in the first trimester, can dissociate TSH from actual thyroid hormone levels 3, 5.
Do not treat with antithyroid drugs if this is gestational transient thyrotoxicosis or postpartum thyroiditis, as these are destructive processes, not excessive hormone production 1, 6.
Do not use radioactive iodine (I-131), which is absolutely contraindicated during pregnancy 1.
Do not overlook the need for fetal monitoring if hyperthyroidism is confirmed, as untreated maternal hyperthyroidism increases risks of severe preeclampsia, preterm delivery, heart failure, miscarriage, and low birth weight 1.
Special Considerations for Third Trimester
Recognize that thyroid hormone demands remain elevated until term due to transplacental passage of thyroid hormones and increased turnover of maternal T4 under the influence of placental type III deiodinase 5.
Understand that FT4 levels remain relatively uniform throughout gestation in healthy pregnant women, with only marginal decreases in iodine-sufficient conditions 5, 2.
Be aware that TSH shows an increasing trend from first to third trimester 2, so a "normal" TSH in the third trimester may actually be at the higher end of the pregnancy-specific range.
Postpartum Management
Inform the newborn's physician about any maternal thyroid disease due to risk of neonatal thyroid dysfunction 1.
Reassure that women treated with PTU or methimazole can breastfeed safely 1.
Monitor for postpartum thyroiditis, which occurs in up to 10% of women and typically presents 1-6 months postpartum with transient hyperthyroidism followed by hypothyroidism 6.