A 41-year-old woman is evaluated in the clinic for progressive fatigue, cold intolerance, … — USMLE Step 2 CK

A 41-year-old woman is evaluated in the clinic for progressive fatigue, cold intolerance, and a 6-kg (13-lb) weight gain over the past eight months. She has also experienced diffuse muscle aches and secondary amenorrhea during this time. A urine pregnancy test is negative. She takes no medications and has no significant past medical history. Vital signs show a blood pressure of $102/64$ mmHg and a pulse of $54$ /min. Physical examination reveals dry skin, a pale appearance, and delayed relaxation of the Achilles tendon reflexes. Palpation of the anterior neck reveals no thyromegaly or nodules.

Laboratory results are as follows:

Parameter	Result	Reference Range
Sodium	$133$ mEq/L	$136 - 145$ mEq/L
Potassium	$4.2$ mEq/L	$3.5 - 5.0$ mEq/L
Thyroid-stimulating hormone (TSH)	$1.4$ $\mu$ U/mL	$0.5 - 5.0$ $\mu$ U/mL
Free thyroxine (T4)	$0.5$ ng/dL	$0.9 - 1.7$ ng/dL

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

Measurement of early morning serum cortisolAnswer
B
Start oral levothyroxine replacement therapy
C
Measurement of anti-thyroid peroxidase antibodies
D
Ultrasound evaluation of the anterior neck
E
Reassurance and repeat thyroid function tests in 6 weeks

Answer

Measurement of early morning serum cortisol

The correct action is to measure an early morning serum cortisol level. This patient's laboratory values (low free T4, normal TSH) indicate central hypothyroidism. Because central hypothyroidism often coexists with other pituitary hormone deficiencies (panhypopituitarism), secondary adrenal insufficiency must be ruled out. Starting levothyroxine increases cortisol clearance; if cortisol is already low and cannot be appropriately upregulated by the damaged pituitary, the patient will be thrust into a fatal adrenal crisis. Therefore, assessing the adrenal axis takes immediate precedence before thyroid replacement.

Step-by-Step Solution

Interpret the thyroid function tests to determine the primary site of pathology.

The patient has a frankly low free T4 combined with a TSH level that is within the normal reference range. This confirms central (secondary) hypothyroidism.

In primary hypothyroidism, the pituitary responds to a low T4 by drastically increasing TSH production. An inappropriately normal or low TSH in the setting of low T4 indicates that the pituitary gland or hypothalamus is failing.

Correlate the central endocrine defect with the clinical presentation.

The patient's central hypothyroidism, coupled with secondary amenorrhea and mild hyponatremia, suggests panhypopituitarism (deficiency of multiple anterior pituitary hormones including TSH, FSH/LH, and potentially ACTH).

Pituitary pathology (such as a non-functioning macroadenoma) rarely affects only a single hormone axis. The mild hyponatremia and low-normal blood pressure raise a high suspicion for concurrent secondary adrenal insufficiency due to ACTH deficiency.

Determine the safest sequential step for therapeutic management.

The adrenal axis must be evaluated by measuring an early morning serum cortisol level before any thyroid replacement is administered.

Initiating levothyroxine accelerates the body's metabolic rate and enhances the metabolism and clearance of cortisol. If a patient with central hypothyroidism has undiagnosed adrenal insufficiency, starting thyroid hormone without concurrent glucocorticoid replacement can rapidly precipitate a life-threatening adrenal crisis.

Key Concept

In patients diagnosed with central hypothyroidism (low T4 with inappropriately normal/low TSH), the hypothalamic-pituitary-adrenal axis must be evaluated (via early morning serum cortisol) before initiating levothyroxine therapy to prevent precipitating an acute adrenal crisis.

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic