Immediate TSH Measurement is the Critical Next Step
In a postpartum patient with signs/symptoms of hypothyroidism, inability to breastfeed, and low T4 (4), the best next step is to measure TSH (Option B) to distinguish between primary hypothyroidism and Sheehan syndrome (postpartum pituitary necrosis). 1, 2
Clinical Reasoning: Why TSH Must Be Measured First
This clinical presentation raises immediate concern for Sheehan syndrome (postpartum pituitary infarction), which presents with:
- Inability to lactate/breastfeed after delivery (failure of prolactin secretion) 1
- Hypothyroid symptoms with low T4 2
- Potential concurrent adrenal insufficiency 1
The Critical Diagnostic Algorithm
Step 1: Measure TSH to determine the etiology 1, 2
If TSH is elevated (>4.5 mIU/L): This indicates primary hypothyroidism (thyroid gland failure). The pituitary is functioning normally and appropriately increasing TSH in response to low T4. 1, 3
If TSH is low or inappropriately normal: This indicates central (secondary) hypothyroidism from pituitary failure (Sheehan syndrome). The pituitary cannot produce adequate TSH despite low T4. 1, 2
Step 2: Management depends entirely on TSH result
If Primary Hypothyroidism (High TSH):
- Start levothyroxine 1.6 mcg/kg/day immediately 2, 4
- No need to rule out adrenal insufficiency 1
- Monitor TSH every 6-8 weeks until normalized 1, 2
If Central Hypothyroidism/Sheehan Syndrome (Low/Normal TSH):
- NEVER start levothyroxine first 1
- Must rule out concurrent adrenal insufficiency immediately by checking morning cortisol and ACTH 1
- If adrenal insufficiency present, start hydrocortisone FIRST (at least 1 week before levothyroxine) to prevent life-threatening adrenal crisis 1
- Only after cortisol replacement, start levothyroxine and titrate based on free T4 levels (not TSH) 2
- Pituitary imaging is indicated to assess extent of pituitary damage 1
Why the Other Options Are Incorrect
Option A (Give 50 mcg thyroxine immediately): This is dangerous without knowing TSH. If this is Sheehan syndrome with concurrent adrenal insufficiency, starting thyroid hormone before corticosteroids can precipitate fatal adrenal crisis. 1
Option C (CBC): While potentially useful for general assessment, it does not address the urgent diagnostic question of primary vs. central hypothyroidism. 1
Option D (No pituitary imaging): This is premature. If TSH is low/normal with low T4 (indicating Sheehan syndrome), pituitary imaging IS indicated to assess the extent of pituitary damage. 1
Critical Red Flags in This Case
The inability to breastfeed is the key clinical clue suggesting pituitary failure rather than primary thyroid disease. 1 This symptom, combined with postpartum timing and hypothyroidism, creates a high pretest probability for Sheehan syndrome. 1
Common Pitfall to Avoid
Never assume all postpartum hypothyroidism is primary thyroid disease. The combination of failure to lactate + hypothyroid symptoms demands evaluation for pituitary failure. Starting levothyroxine empirically without measuring TSH risks missing Sheehan syndrome and potentially causing adrenal crisis if concurrent adrenal insufficiency exists. 1