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Difficulty: Very hardNeurocognitive Disorders

A 72-year-old woman is brought to the clinic by her son due to a 6-month history of progressive memory impairment, apathy, and severe psychomotor slowing. She frequently forgets conversations and has stopped participating in her usual social activities. She also reports worsening fatigue, constipation, and feeling cold constantly. Her medical history is significant for bipolar I disorder, which has been well-controlled on lithium monotherapy for 15 years, and hypertension. Current medications are lithium and lisinopril. Vital signs show a blood pressure of 130/80 mm Hg and a heart rate of 52/min. Physical examination reveals delayed relaxation of the deep tendon reflexes, dry skin, and a slightly puffy face. Cognitive assessment yields a Montreal Cognitive Assessment (MoCA) score of 19/30.

Laboratory results are as follows:

TestResultReference Range
Serum lithium0.8 mEq/L0.6-1.2 mEq/L
Thyroid-stimulating hormone (TSH)0.2 mIU/L0.4-4.0 mIU/L
Free thyroxine (T4)0.5 ng/dL0.9-1.7 ng/dL

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

  1. Magnetic resonance imaging of the brainAnswer
  2. B
    Measurement of serum anti-thyroid peroxidase antibodies
  3. C
    Discontinuation of lithium and initiation of valproate
  4. D
    Initiation of concurrent donepezil and amitriptyline
  5. E
    Discontinuation of lithium and initiation of sertraline

Answer

Magnetic resonance imaging of the brain
This patient presents with a major neurocognitive decline accompanied by classical signs of hypothyroidism (bradycardia, delayed deep tendon reflexes, constipation, cold intolerance). In the workup of a neurocognitive disorder, evaluating for reversible causes is mandatory. Her laboratory findings reveal a low free T4 level, confirming hypothyroidism. However, her thyroid-stimulating hormone (TSH) level is inappropriately low. In primary hypothyroidism, the lack of negative feedback from T4 leads to a dramatically elevated TSH. A low or inappropriately normal TSH in the setting of low free T4 indicates secondary (central) hypothyroidism, pointing to a failure of the pituitary gland or hypothalamus. The most common causes of central hypothyroidism are structural lesions, such as a pituitary macroadenoma, craniopharyngioma, or other compressive intracranial masses. Therefore, the most appropriate next step in management is a magnetic resonance imaging (MRI) scan of the brain to visualize the hypothalamic-pituitary axis.

Step-by-Step Solution

1
Identify the primary clinical syndrome.
The patient presents with reversible neurocognitive impairment (pseudodementia) accompanied by classic hypothyroid signs (bradycardia, delayed reflexes, cold intolerance, constipation).
Establishing the clinical phenotype is essential before interpreting laboratory values in the context of cognitive decline.
2
Interpret the thyroid function tests.
The free T4 is low (confirming hypothyroidism), but the TSH is inappropriately low (0.2 mIU/L), establishing a diagnosis of secondary (central) hypothyroidism.
Differentiating primary (high TSH) from secondary (low/normal TSH) hypothyroidism determines the anatomic location of the defect.
3
Differentiate the etiology from medication adverse effects.
Lithium induces primary hypothyroidism by inhibiting thyroid hormone release, which would elevate TSH. The patient's low TSH pattern rules out lithium-induced thyroid dysfunction.
Ruling out medication-induced effects prevents inappropriate cessation of a stable, long-term psychiatric regimen.
4
Determine the appropriate diagnostic intervention for central hypothyroidism.
Magnetic resonance imaging (MRI) of the brain is required.
Central hypothyroidism is most frequently caused by structural compressive lesions in the hypothalamic-pituitary axis (e.g., macroadenoma), which must be visualized radiographically.

Key Concept

Reversible neurocognitive impairment can be caused by central hypothyroidism (characterized by low free T4 and an inappropriately low TSH), which requires an MRI of the brain to evaluate for a pituitary or hypothalamic mass, independent of chronic lithium therapy which typically causes primary hypothyroidism (high TSH).
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Neurocognitive Disorders

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