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Difficulty: MediumEndocrinology and Metabolism

A 3131-year-old woman comes to the clinic for follow-up of palpitations, generalized anxiety, and heat intolerance that began 33 weeks ago. She delivered a healthy infant 44 months ago. She has no significant past medical history and takes no medications. Her temperature is 37.1∘C37.1^\circ\text{C} (98.8∘F98.8^\circ\text{F}), blood pressure is 128/78 mmHg128/78\text{ mmHg}, and pulse is 112/min112\text{/min}. Physical examination reveals a mild, diffusely enlarged, non-tender thyroid gland. There is no proptosis, lid lag, or pretibial myxedema. Laboratory results show:

TestResultReference Range
Thyroid-stimulating hormone (TSH)0.05 mU/L0.05\text{ mU/L}0.50.5-5.0 mU/L5.0\text{ mU/L}
Free thyroxine (T4)2.4 ng/dL2.4\text{ ng/dL}0.90.9-1.7 ng/dL1.7\text{ ng/dL}

A radioactive iodine uptake (RAIU) scan demonstrates 2%2\% uptake at 2424 hours (normal: 10%10\%-30%30\%).

Which of the following is the most appropriate next step in management?

  1. Administration of propranololAnswer
  2. B
    Initiation of methimazole
  3. C
    Administration of prednisone
  4. D
    Referral for radioactive iodine ablation
  5. E
    Initiation of levothyroxine

Answer

Administration of propranolol is the most appropriate next step.
The correct answer is the administration of propranolol. This patient's presentation of thyrotoxicosis (palpitations, tachycardia, heat intolerance, low TSH, high Free T4) within one year of delivery, accompanied by a painless goiter and a low radioactive iodine uptake (RAIU), is classic for the hyperthyroid phase of postpartum thyroiditis. Postpartum thyroiditis is a variant of painless (silent) autoimmune thyroiditis. The low RAIU confirms that the excess circulating thyroid hormone is due to the destruction of thyroid follicles and release of preformed hormone, rather than de novo synthesis. Because this phase is self-limited and not driven by overproduction, antithyroid synthesis inhibitors are ineffective. The standard of care is symptomatic management of hyperadrenergic symptoms using a beta-blocker (e.g., propranolol) until the hyperthyroid phase resolves.

Step-by-Step Solution

1
Interpret the clinical presentation and initial laboratory findings.
The patient has symptoms of thyrotoxicosis (tachycardia, palpitations, heat intolerance) beginning after a recent pregnancy, supported by an elevated Free T4 and a suppressed TSH.
Establishing the biochemical presence of thyrotoxicosis is the required first step before determining the etiology.
2
Analyze the radioactive iodine uptake (RAIU) scan results.
The RAIU scan shows severely decreased uptake (2%2\%).
A low RAIU distinguishes destructive thyroiditis (release of preformed hormone) from conditions of hormone overproduction (like Graves disease, which would have high uptake).
3
Synthesize findings to determine the correct diagnosis and treatment.
A painless goiter in the postpartum period with hyperthyroid labs and a low RAIU indicates the hyperthyroid phase of postpartum thyroiditis. Symptom control with a beta-blocker is indicated.
Because postpartum thyroiditis is a transient destructive process, antithyroid medications are ineffective. Beta-blockers relieve the hyperadrenergic symptoms until the phase resolves.

Key Concept

Postpartum Thyroiditis Diagnostics and Management
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Topics

Endocrinology and Metabolism

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