An outpatient evaluation is conducted for a 35-year-old female patient who reports severe … — USMLE Step 2 CK

An outpatient evaluation is conducted for a $35$ -year-old female patient who reports severe fatigue and feeling constantly cold. Over the past $6$ months, she has noticed a $5\text{-kg}$ ( $11\text{-lb}$ ) weight gain despite a poor appetite, and her menstrual cycles have completely stopped. She has no chronic medical conditions and takes no medications. Vital signs show a temperature of $36.2^\circ\text{C}$ ( $97.2^\circ\text{F}$ ), blood pressure of $108/68 \text{ mm Hg}$ , and pulse of $52/\text{min}$ . Physical examination reveals dry skin, delayed relaxation of the deep tendon reflexes, and no palpable thyroid enlargement. Visual fields are full to confrontation. Laboratory results are as follows:

Thyroid-stimulating hormone (TSH): $0.1 \text{ mU/L}$ (Normal: $0.4 - 4.0 \text{ mU/L}$ )
Free thyroxine (T4): $0.4 \text{ ng/dL}$ (Normal: $0.9 - 1.7 \text{ ng/dL}$ )

Which of the following is the most likely underlying cause of this patient's clinical presentation?

Anterior pituitary adenomaAnswer
B
Autoimmune destruction of the thyroid gland
C
Surreptitious use of levothyroxine
D
Decreased peripheral conversion of thyroxine
E
Granulomatous inflammation of the thyroid gland

Answer

The most likely diagnosis is an anterior pituitary adenoma, which causes secondary hypothyroidism and hypogonadotropic hypogonadism.

The correct answer accurately synthesizes the laboratory data (low free T4 and low TSH) to localize the defect to the central nervous system (pituitary or hypothalamus) rather than the thyroid gland itself. The concurrent history of amenorrhea makes an anterior pituitary adenoma the most likely unifying diagnosis, as expanding pituitary lesions frequently compress adjacent normal pituitary tissue, leading to multiple anterior pituitary hormone deficiencies (panhypopituitarism).

Step-by-Step Solution

Interpret the clinical symptoms and vital signs.

The patient exhibits weight gain, fatigue, cold intolerance, delayed reflexes, and bradycardia, establishing a clinical hypothyroid state.

Initial pattern recognition of the metabolic state directs further laboratory analysis.

Analyze the thyroid function laboratory tests.

The free T4 is low, confirming hypothyroidism, while the TSH is inappropriately low. This combination rules out a primary thyroid gland defect and indicates central (secondary) hypothyroidism.

The relationship between TSH and T4 determines the anatomical location of the defect in the hypothalamic-pituitary-thyroid axis.

Correlate the laboratory diagnosis with the secondary symptom of amenorrhea.

The combination of central hypothyroidism and amenorrhea points to a widespread disruption of the anterior pituitary gland.

A pituitary adenoma can cause a mass effect or hyperprolactinemia, which simultaneously disrupts the secretion of both TSH and gonadotropins (FSH/LH).

Key Concept

Secondary (central) hypothyroidism is characterized by a low free T4 and a low or inappropriately normal TSH. When accompanied by other pituitary hormone deficiencies (such as amenorrhea), it strongly suggests a pituitary lesion like an adenoma.

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