A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 34-year-old woman presents with fatigue, depressed mood, weight gain, and constipation. She gradually developed these symptoms over the past 6 months. She is G2P2 with the last pregnancy 9 months ago. She had a complicated delivery with significant blood loss requiring blood transfusions. She used to have a regular 28-day cycle but notes that recently it became irregular with duration lasting up to 40 days, more pain, and greater blood loss. She does not report any chronic conditions, and she is not on any medications. She is a current smoker with a 10-pack-year history. Her blood pressure is 130/80 mm Hg, heart rate is 54/min, respiratory rate is 11/min, and temperature is 35.8°C (96.4°F). Her skin is dry and pale with a fine scaling over the forearms and shins. There is a mild, non-pitting edema of the lower legs. Her lungs are clear to auscultation. Cardiac auscultation does not reveal any pathological sounds or murmurs although S1 and S2 are dulled at all points of auscultation. The abdomen is mildly distended and nontender on palpation. Neurological examination is significant for decreased deep tendon reflexes. Her blood tests show the following results:
Erythrocytes count 3.4 million/mm3
Hb 12.2 mg/dL
MCV 90 μm3
Reticulocyte count 0.3%
Leukocyte count 5,600/mm3
Serum vitamin B12 210 ng/mL
T4 total 1.01 μU/mL
T4 free 0.6 μU/mL
TSH 0.2 μU/mL
Which of the following lab values should be used to monitor treatment in this patient?
-
A
Free T4Correct. in central hypothyroidism (Sheehan syndrome with inappropriately low TSH despite low T4), free T4 is the only reliable marker for monitoring levothyroxine therapy, since TSH cannot be used.
-
B
Total T3Incorrect. Total T3 is not the standard monitoring parameter; levothyroxine replaces T4, which is then peripherally converted to T3, and free T4 better reflects replacement adequacy.
-
C
MCVIncorrect. MCV monitors megaloblastic anemia treatment (B12, folate) — not thyroid replacement.
-
D
TSHIncorrect. TSH is the monitoring parameter in PRIMARY hypothyroidism but is unreliable in central hypothyroidism (Sheehan), where pituitary dysfunction prevents an appropriate TSH response.
-
E
Serum prolactinIncorrect. Prolactin levels can be low in Sheehan syndrome and help establish the diagnosis, but they do not monitor adequacy of levothyroxine replacement, which requires free T4.
↑ Tap an answer to reveal the reasoning
Answer: A. A postpartum woman 9 months after a complicated delivery with significant hemorrhage who developed amenorrhea/irregular menses, fatigue, weight gain, constipation, cold intolerance, bradycardia, hypothermia, dry pale skin, non-pitting edema (myxedema), and decreased reflexes has SHEEHAN SYNDROME — pituitary infarction from postpartum hemorrhage-induced hypoperfusion of the enlarged pituitary. The hormonal pattern (low free T4 with INAPPROPRIATELY LOW/normal TSH) is the hallmark of CENTRAL hypothyroidism — the pituitary cannot mount an appropriate TSH response despite low thyroid hormone.
This is critically important: in PRIMARY hypothyroidism, TSH is high and treatment is monitored by TSH normalization. In CENTRAL/SECONDARY hypothyroidism (as in Sheehan), TSH is suppressed/normal and is NOT a reliable marker for therapy adjustment — instead, FREE T4 must be used to guide levothyroxine dosing, targeting the mid-to-upper normal range.
Management also requires evaluating other pituitary axes (ACTH/cortisol must be replaced BEFORE thyroid hormone to avoid precipitating adrenal crisis, plus FSH/LH, prolactin — failed lactation is the early Sheehan clue, GH).
Not Total T3 (less useful for monitoring), not MCV (not relevant), not TSH (suppressed in central disease).
**Why each option:**
**A.** Correct — in central hypothyroidism (Sheehan syndrome with inappropriately low TSH despite low T4), free T4 is the only reliable marker for monitoring levothyroxine therapy, since TSH cannot be used.
**B.** Total T3 is not the standard monitoring parameter; levothyroxine replaces T4, which is then peripherally converted to T3, and free T4 better reflects replacement adequacy.
**C.** MCV monitors megaloblastic anemia treatment (B12, folate) — not thyroid replacement.
**D.** TSH is the monitoring parameter in PRIMARY hypothyroidism but is unreliable in central hypothyroidism (Sheehan), where pituitary dysfunction prevents an appropriate TSH response.