A 45-year-old woman comes to the clinic reporting progressive fatigue, generalized weaknes… — USMLE Step 2 CK

A 45-year-old woman comes to the clinic reporting progressive fatigue, generalized weakness, and constipation over the past 4 months. She also reports a $4\text{-kg}$ ( $8.8\text{-lb}$ ) weight gain during this time despite a decreased appetite. Her menstrual periods have been absent for the past 6 months. Her only medication is a levonorgestrel-releasing intrauterine device. Blood pressure is $105/65\text{ mm Hg}$ , and pulse is $58\text{/min}$ . Physical examination shows dry skin and delayed relaxation of the deep tendon reflexes. Visual fields are full to confrontation. Laboratory studies show:

Laboratory Test	Result	Reference Range
Thyroid-stimulating hormone (TSH)	$1.4 \text{ \mu U/mL}$	$0.5-5.0 \text{ \mu U/mL}$
Free thyroxine (Free T4)	$0.3 \text{ ng/dL}$	$0.9-1.7 \text{ ng/dL}$

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

Measurement of early morning serum cortisolAnswer
B
Initiation of levothyroxine therapy
C
Measurement of anti-thyroid peroxidase antibodies
D
Reassurance and repeat laboratory testing in 6 weeks
E
Ultrasound of the thyroid gland

Answer

Measurement of early morning serum cortisol

The correct next step is the measurement of early morning serum cortisol. This patient presents with classic symptoms of hypothyroidism (fatigue, weight gain, delayed reflexes) coupled with a low free T4 and an inappropriately normal TSH. This pattern is diagnostic of central (secondary) hypothyroidism. Furthermore, her amenorrhea suggests concurrent hypogonadotropic hypogonadism, pointing toward a broader panhypopituitarism. The most critical step in managing central hypothyroidism is assessing the hypothalamic-pituitary-adrenal (HPA) axis prior to thyroid replacement. Thyroid hormone increases the hepatic metabolic clearance of cortisol. If levothyroxine is administered to a patient with unrecognized adrenal insufficiency, it can deplete their minimal cortisol reserves and precipitate a life-threatening adrenal crisis.

Step-by-Step Solution

Interpret the thyroid function tests.

The patient has a low free T4 (

0.3\text{ ng/dL}

) with an inappropriately normal TSH (

1.4\text{ \mu U/mL}

Identifying the pattern of low peripheral hormone without an appropriate central stimulating hormone response localizes the pathology to the pituitary or hypothalamus (central hypothyroidism).

Identify other potential endocrine abnormalities based on the clinical presentation.

The patient also has a 6-month history of secondary amenorrhea.

Amenorrhea in the setting of central hypothyroidism strongly suggests panhypopituitarism (multiple anterior pituitary hormone deficiencies).

Determine the safest sequence of management for panhypopituitarism.

The adrenal axis must be evaluated (with a morning cortisol or ACTH stimulation test) before replacing thyroid hormone.

Thyroid hormone increases the metabolic clearance of glucocorticoids. Initiating levothyroxine in a patient with undiagnosed central adrenal insufficiency can rapidly precipitate a fatal adrenal crisis.

Key Concept

In patients diagnosed with central hypothyroidism, central adrenal insufficiency must be ruled out (e.g., via early morning cortisol) before initiating levothyroxine therapy to avoid precipitating an adrenal crisis.

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