A 28-year-old nulligravid woman presents to the clinic alongside her partner, reporting an… — USMLE Step 2 CK

A 28-year-old nulligravid woman presents to the clinic alongside her partner, reporting an inability to conceive after 12 months of regular, unprotected intercourse. She previously experienced predictable 28-day menstrual cycles, but over the past eight months, she has had only two episodes of light vaginal bleeding. The patient notes a generalized lack of energy and an unintentional 8-kg (17.6-lb) weight gain during this period. She also reports the recent appearance of dark, coarse hair on her chin. Her body mass index is 31 kg/m $^2$ . Vital signs show a heart rate of 56/min and blood pressure of 112/74 mmHg. Pelvic examination is unremarkable. Transvaginal ultrasonography reveals symmetrically enlarged ovaries, each containing 15 to 20 small subcapsular follicles.

Laboratory studies show the following:
- $\beta$ -hCG: negative
- Total testosterone: 65 ng/dL (Reference: 15-70 ng/dL)
- Prolactin: 48 ng/mL (Reference: < 20 ng/mL)
- Thyroid-stimulating hormone (TSH): 11.2 $\mu$ U/mL (Reference: 0.4-4.0 $\mu$ U/mL)
- Free thyroxine (T4): 0.6 ng/dL (Reference: 0.9-1.7 ng/dL)
- Follicle-stimulating hormone (FSH): 4 mIU/mL
- Luteinizing hormone (LH): 3 mIU/mL

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

Initiate levothyroxine supplementationAnswer
B
Start therapy with an aromatase inhibitor
C
Order neuroimaging of the sella turcica
D
Prescribe a dopamine agonist
E
Measure early morning serum 17-hydroxyprogesterone

Answer

Initiate levothyroxine supplementation

The correct answer is to initiate levothyroxine supplementation. This patient has primary hypothyroidism, evidenced by an elevated TSH and low free T4. Primary hypothyroidism can perfectly mimic Polycystic Ovary Syndrome (PCOS). The lack of thyroid hormone negative feedback causes the hypothalamus to increase thyrotropin-releasing hormone (TRH) secretion. TRH acts on the anterior pituitary to stimulate not only TSH but also prolactin. The resulting mild hyperprolactinemia suppresses gonadotropin-releasing hormone (GnRH) pulsatility, leading to hypogonadotropic hypogonadism, amenorrhea, and anovulatory infertility. Furthermore, hypothyroidism reduces the hepatic synthesis of sex hormone-binding globulin (SHBG). This leads to an increase in free, biologically active testosterone, which causes clinical hirsutism even when total testosterone is within the normal reference range. Finally, the chronic anovulation results in the polycystic ovarian morphology observed on ultrasound. Because her entire reproductive dysfunction is driven by the thyroid deficit, the most appropriate next step is to initiate levothyroxine. This will normalize TRH and prolactin levels, increase SHBG, and typically restore spontaneous ovulation and fertility.

Step-by-Step Solution

Analyze the clinical presentation and ultrasound findings.

The patient has secondary amenorrhea, weight gain, fatigue, mild hirsutism, and polycystic ovarian morphology (PCOM).

This constellation of findings strongly mimics Polycystic Ovary Syndrome (PCOS), but requires a review of the endocrine laboratory panel to rule out other etiologies.

Interpret the thyroid and prolactin laboratory values.

TSH is markedly elevated and free T4 is low, diagnosing primary hypothyroidism. Prolactin is mildly elevated.

In primary hypothyroidism, the lack of negative feedback from T4 leads to increased thyrotropin-releasing hormone (TRH) from the hypothalamus. TRH stimulates both thyrotrophs (increasing TSH) and lactotrophs (increasing prolactin).

Connect the physiological mechanism to the reproductive symptoms.

Elevated prolactin suppresses GnRH pulsatility, leading to anovulation and amenorrhea. Additionally, hypothyroidism decreases hepatic production of sex hormone-binding globulin (SHBG).

Decreased SHBG increases the proportion of unbound, biologically active (free) testosterone, explaining the hirsutism even with a normal total testosterone level. Chronic anovulation leads to the PCOM seen on ultrasound.

Determine the next best step in management.

Treat the underlying primary hypothyroidism with levothyroxine.

Restoring euthyroidism will decrease TRH, normalize prolactin, restore normal GnRH pulsatility, and typically resolve the anovulatory infertility without the need for ovulation induction agents.

Key Concept

Primary hypothyroidism can cause a reactive hyperprolactinemia (due to TRH stimulation) and decreased SHBG, creating a clinical picture that perfectly mimics PCOS (amenorrhea, hirsutism, polycystic ovaries).

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