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Difficulty: EasyMedical History Evaluation

A 4545-year-old woman comes to the ambulatory clinic complaining of a three-month history of worsening fatigue, a 4.54.5-kg (1010-lb) weight gain, and new-onset constipation. Her vital signs include a temperature of 36.8∘36.8^\circ C (98.2∘98.2^\circ F), blood pressure of 118/76118/76 mmHg, and heart rate of 5858/min. Physical examination reveals dry skin and delayed relaxation of the deep tendon reflexes. Laboratory testing shows a thyroid-stimulating hormone (TSH) level of 12.412.4 mU/L (normal: 0.4−4.00.4-4.0 mU/L) and a free thyroxine (T4) level of 0.60.6 ng/dL (normal: 0.9−1.70.9-1.7 ng/dL). The physician carefully reviews the patient's medical and medication history to determine the underlying cause of her presentation. Which of the following historical findings is the most likely etiology for this patient's current condition?

  1. Long-term use of lithium carbonate for a psychiatric conditionAnswer
  2. B
    Recent initiation of combined oral contraceptive pills
  3. C
    Regular consumption of over-the-counter St. John's wort
  4. D
    A viral upper respiratory tract infection two weeks ago
  5. E
    Family history of medullary thyroid carcinoma

Answer

Long-term use of lithium carbonate for a psychiatric condition
This patient exhibits classic signs and laboratory findings of primary hypothyroidism, specifically an elevated TSH and a low free T4, alongside fatigue, weight gain, delayed reflexes, and bradycardia. When evaluating the medical history for etiologies of new-onset primary hypothyroidism, a history of lithium carbonate use is a critical finding. Lithium inhibits the release of thyroid hormone from the thyroid gland and can lead to clinical hypothyroidism. For this reason, patients on long-term lithium therapy require routine monitoring of their thyroid function.

Step-by-Step Solution

1
Interpret the clinical and laboratory findings.
The patient has symptomatic bradycardia, weight gain, fatigue, an elevated TSH, and a low free T4.
These findings definitively establish the diagnosis of primary hypothyroidism.
2
Evaluate the medical history to identify potential etiologies for the established diagnosis.
Reviewing medication history is a critical step in working up new-onset primary hypothyroidism to identify iatrogenic causes.
Certain medications directly interfere with thyroid hormone synthesis or release.
3
Match the correct historical exposure to the pathophysiology of primary hypothyroidism.
Lithium carbonate is identified as the causal agent.
Lithium is a common drug that inhibits thyroid hormone release, necessitating routine TSH monitoring for patients on this therapy.

Key Concept

Medication-induced primary hypothyroidism (such as that caused by lithium or amiodarone) must be evaluated during the medical history review of a patient presenting with an elevated TSH and low free T4.
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Medical History Evaluation

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