A 34-year-old woman comes to the outpatient medical clinic reporting a 6-month history of … — USMLE Step 2 CK

A $34$ -year-old woman comes to the outpatient medical clinic reporting a $6$ -month history of progressive fatigue, weakness, and a $4.5\ \text{kg}$ ( $10\ \text{lb}$ ) weight gain. She notes that her menstrual periods, which were previously regular, have ceased entirely over the same timeframe. She takes no medications and has no other medical history. Vital signs show a temperature of $36.4^\circ\text{C}$ ( $97.5^\circ\text{F}$ ), pulse of $54/\text{min}$ , and blood pressure of $100/65\ \text{mm Hg}$ . Physical examination reveals dry skin and a delayed relaxation phase of the deep tendon reflexes. The thyroid gland is not palpable. A urine pregnancy test is negative.

Laboratory results are as follows:

Laboratory Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$1.2\ \mu\text{U/mL}$	$0.5-5.0\ \mu\text{U/mL}$
Free thyroxine (T4)	$0.4\ \text{ng/dL}$	$0.9-2.4\ \text{ng/dL}$

Which of the following is the most appropriate next step in the evaluation of this patient?

Magnetic resonance imaging of the brainAnswer
B
Initiation of levothyroxine therapy
C
Measurement of anti-thyroid peroxidase antibodies
D
Ultrasound of the thyroid gland
E
Reassurance and repeat laboratory testing in 6 weeks

Answer

Magnetic resonance imaging of the brain is the most appropriate next step to evaluate for a structural pituitary lesion causing central hypothyroidism.

The correct answer is magnetic resonance imaging of the brain. The patient's presentation of fatigue, weight gain, bradycardia, and delayed reflexes strongly suggests hypothyroidism, which is confirmed by the low free T4. However, her TSH is inappropriately normal. In primary hypothyroidism (thyroid gland failure), the loss of negative feedback leads to a high TSH. A low T4 with a low or normal TSH indicates central hypothyroidism, which stems from insufficient TSH production by the anterior pituitary gland or insufficient TRH from the hypothalamus. Her concurrent amenorrhea indicates a deficiency in gonadotropins (LH/FSH), further suggesting a compressive structural lesion in the pituitary, such as a macroadenoma. Therefore, the best next step is an MRI of the brain to visualize the sella turcica.

Step-by-Step Solution

Interpret the thyroid function tests in the context of the patient's symptoms.

The patient has clinical hypothyroidism with a low free T4, but the TSH is inappropriately normal rather than elevated.

This pattern distinguishes central (secondary) hypothyroidism from primary hypothyroidism, indicating a failure of the pituitary or hypothalamus.

Synthesize the laboratory findings with the clinical history of amenorrhea.

The combination of central hypothyroidism and secondary amenorrhea points to a global anterior pituitary dysfunction, likely due to a mass lesion.

Multiple anterior pituitary hormone deficiencies strongly suggest structural pituitary disease, such as a non-functioning pituitary adenoma or prolactinoma.

Determine the most appropriate next step in evaluation.

Order a magnetic resonance imaging (MRI) of the brain.

An MRI is required to visualize the pituitary gland and hypothalamus to identify any structural causes before initiating hormone replacement therapy.

Key Concept

Central (secondary) hypothyroidism is characterized by a low free T4 and an inappropriately normal or low TSH. Evaluation requires an MRI of the brain to rule out structural pituitary lesions, and concurrent adrenal insufficiency must be excluded before starting levothyroxine.

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