A 52-year-old woman is evaluated in the surgical clinic for a progressively enlarging ante… — USMLE Step 2 CK

A $52$ -year-old woman is evaluated in the surgical clinic for a progressively enlarging anterior neck mass. Over the past $6$ months, she has noticed difficulty swallowing solid foods and a feeling of pressure in her neck when lying flat. During this time, she has also experienced profound fatigue, a $4$ -kg ( $8.8$ -lb) weight gain, and occasional morning headaches. She has no prior medical history and takes no medications. Vital signs are temperature $36.8^\circ$ C ( $98.2^\circ$ F), blood pressure $110/70$ mm Hg, pulse $64$ /min, and respirations $14$ /min. Physical examination reveals a large, firm, multinodular goiter. The lower border of the thyroid cannot be palpated above the sternal notch. A CT scan of the neck and chest without contrast demonstrates a massive multinodular goiter with significant tracheal deviation and substernal extension.

Preoperative laboratory results are as follows:

Laboratory Test	Result	Reference Range
Serum sodium	$133$ mEq/L	$136-145$ mEq/L
Serum potassium	$4.2$ mEq/L	$3.5-5.0$ mEq/L
Serum calcium	$9.3$ mg/dL	$8.4-10.2$ mg/dL
Thyroid-stimulating hormone (TSH)	$2.2$ $\mu$ U/mL	$0.4-4.0$ $\mu$ U/mL
Free thyroxine (T4)	$0.5$ ng/dL	$0.9-1.7$ ng/dL

The surgeon plans to perform a total thyroidectomy to relieve the patient's compressive symptoms. Which of the following is the most appropriate next step in the management of this patient?

A
Initiation of levothyroxine therapy and scheduling of surgery
B
Measurement of serum free triiodothyronine (T3) level
Measurement of early morning serum cortisol levelAnswer
D
Proceed with total thyroidectomy as planned
E
Administration of empiric intravenous hydrocortisone

Answer

The most appropriate next step is the measurement of an early morning serum cortisol level to evaluate for concomitant secondary adrenal insufficiency.

The correct action is the measurement of an early morning serum cortisol level. This patient has a large substernal goiter requiring total thyroidectomy, but her preoperative labs reveal a low free T4 with an inappropriately normal TSH. This pattern is diagnostic of central (secondary) hypothyroidism, indicating an underlying pituitary or hypothalamic disorder (e.g., nonfunctioning pituitary macroadenoma, which would also explain her mild hyponatremia, fatigue, and morning headaches). In a patient with central hypothyroidism, there is a high probability of concomitant secondary adrenal insufficiency due to decreased adrenocorticotropic hormone (ACTH) secretion. Before subjecting this patient to the major physiologic stress of a total thyroidectomy, her adrenal axis must be evaluated. Furthermore, initiating thyroid hormone replacement in a patient with undiagnosed adrenal insufficiency will increase the hepatic clearance of cortisol, which can precipitate a life-threatening acute adrenal crisis. Therefore, adrenal evaluation and, if necessary, glucocorticoid replacement must precede both thyroid surgery and levothyroxine therapy.

Step-by-Step Solution

Analyze the patient's preoperative thyroid function tests.

Free T4 is low (

0.5

ng/dL), but TSH is inappropriately normal (

2.2

\mu

U/mL).

To determine the etiology of hypothyroidism, as primary hypothyroidism would present with a markedly elevated TSH.

Identify the underlying endocrine pathology.

The lab pattern indicates central (secondary) hypothyroidism, suggesting pituitary or hypothalamic dysfunction.

An inappropriately normal or low TSH in the setting of a low Free T4 is the hallmark of central hypothyroidism, often caused by a pituitary adenoma.

Assess the implications of pituitary dysfunction for surgical planning.

Panhypopituitarism often involves multiple axes; secondary adrenal insufficiency (low ACTH and cortisol) is highly likely and potentially life-threatening.

Surgical stress requires a robust cortisol response. Without it, the patient will develop an acute adrenal crisis during or after surgery.

Determine the most critical next step before surgery or levothyroxine therapy.

Evaluate the hypothalamic-pituitary-adrenal (HPA) axis with a morning serum cortisol level.

Thyroid hormone increases the metabolic clearance of cortisol. Administering levothyroxine or subjecting the patient to surgical stress before confirming and treating adrenal insufficiency can precipitate a fatal adrenal crisis.

Key Concept

Preoperative evaluation of central hypothyroidism and the risk of concomitant secondary adrenal insufficiency.

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