What is the recommended initial test for a postpartum female presenting with heat intolerance, irritability, sweating, tachycardia, and unintentional weight loss?

Thyroid Function Testing in Postpartum Hyperthyroidism

Order a serum TSH level as the initial test, followed by free T4 (FT4) if TSH is suppressed, to diagnose the cause of thyrotoxicosis in this postpartum patient.

Diagnostic Approach

The clinical presentation of heat intolerance, irritability, sweating, tachycardia, and unintentional weight loss strongly suggests thyrotoxicosis in the postpartum period. 1 The key diagnostic challenge is distinguishing between Graves' disease and postpartum thyroiditis, as management differs significantly. 2, 3

Initial Laboratory Testing

• Start with serum TSH measurement - This is the most sensitive screening test for thyroid dysfunction 1
• If TSH is suppressed (typically <0.4 mIU/L), measure free T4 (FT4) and free T3 (T3) to confirm overt hyperthyroidism 1
• The diagnosis of hyperthyroidism is confirmed by suppressed TSH with elevated FT4 or T3 levels 1, 4

Distinguishing the Etiology

Once biochemical thyrotoxicosis is confirmed, differentiate between causes:

• Measure TSH receptor antibodies (TRAb) - Positive results indicate Graves' disease, which accounts for 95% of hyperthyroidism cases in pregnancy and can recur postpartum 1, 3
• Consider thyroid scintigraphy if antibodies are negative or diagnosis unclear - However, this is contraindicated during lactation 1, 2
• Postpartum thyroiditis occurs in 5-7.2% of women in the first year after delivery and presents with a thyrotoxic phase followed by hypothyroidism 3, 5

Clinical Context and Risk Factors

Testing should be prioritized in high-risk postpartum women, including those with:

• Previous postpartum thyroiditis 2
• Known Graves' disease history 1, 3
• Hashimoto's thyroiditis on thyroid hormone replacement 2
• Thyroid peroxidase antibody positivity 2
• Type 1 diabetes mellitus or other autoimmune diseases 2

Critical Management Considerations

Untreated hyperthyroidism carries significant morbidity and mortality risks including cardiac arrhythmias, heart failure, osteoporosis, and increased mortality. 1, 4 In the postpartum setting, thyroid storm—though rare—is a medical emergency requiring immediate recognition. 1

Common Pitfalls to Avoid

• Do not rely on A1C or other non-thyroid tests - The symptoms are classic for thyrotoxicosis and require thyroid-specific testing 1
• Do not delay testing - Symptomatic thyrotoxicosis requires prompt diagnosis to prevent cardiovascular complications 1, 4
• Do not order radioactive iodine studies if the patient is breastfeeding - This is absolutely contraindicated during lactation 2, 3
• Do not assume postpartum thyroiditis without excluding Graves' disease - Treatment differs fundamentally: postpartum thyroiditis requires supportive care while Graves' disease may require antithyroid drugs 2, 3

Lactation Considerations

If treatment becomes necessary and the patient is breastfeeding, levothyroxine (for subsequent hypothyroid phase) and low-dose antithyroid drugs (propylthiouracil or methimazole) can be safely used. 2 However, radioactive iodine treatment is absolutely contraindicated. 2