Treatment for Primary Hypothyroidism with Normal Adrenal Reserve
This patient requires levothyroxine replacement therapy for her primary hypothyroidism, as her ACTH stimulation test demonstrates adequate adrenal reserve and rules out adrenal insufficiency. 1
Interpretation of the ACTH Stimulation Test
The patient's baseline cortisol of 194 nmol/L (approximately 7 μg/dL) is low, but her post-stimulation cortisol of 545 nmol/L (approximately 19.7 μg/dL) exceeds the diagnostic threshold of 500 nmol/L, confirming normal adrenal function and excluding adrenal insufficiency. 1
A peak cortisol >500-550 nmol/L (>18-20 μg/dL) at 30 or 60 minutes after cosyntropin administration is considered a normal response and rules out adrenal insufficiency. 1
The hypotension (BP 96/68 mm Hg) and low baseline cortisol are likely related to the untreated hypothyroidism rather than adrenal insufficiency, as patients with primary hypothyroidism can have blunted baseline cortisol responses that normalize after thyroid hormone replacement. 2
Primary Treatment: Levothyroxine Replacement
Initiate levothyroxine replacement therapy for the primary hypothyroidism as the definitive treatment. 3
For a 22-year-old woman without cardiac disease, start levothyroxine at a full replacement dose of approximately 1.6 mcg/kg/day (typically 100-125 mcg daily for most young adults). 3
Administer levothyroxine as a single daily dose on an empty stomach, one-half to one hour before breakfast with a full glass of water. 3
Titrate the dosage by 12.5 to 25 mcg increments every 4 to 6 weeks based on serum TSH levels until the patient is clinically euthyroid and TSH normalizes. 3
Why Glucocorticoid Replacement is NOT Indicated
The adequate cortisol response to ACTH stimulation (545 nmol/L) definitively excludes adrenal insufficiency, making glucocorticoid replacement unnecessary and potentially harmful. 1
Research demonstrates that 6.7-18.3% of patients with untreated primary hypothyroidism have impaired cortisol responses on ACTH stimulation testing, but more than 50% of these cases normalize after achieving euthyroidism with levothyroxine therapy alone. 2
This patient's low baseline cortisol likely represents the reversible adrenal dysfunction associated with hypothyroidism rather than true adrenal insufficiency, given her normal stimulated response. 2
Critical Monitoring and Follow-up
Recheck TSH and free T4 levels 4-6 weeks after initiating levothyroxine, as the peak therapeutic effect may not be attained for 4 to 6 weeks. 3
Monitor blood pressure at follow-up visits, as hypotension should improve with thyroid hormone replacement. 4
If the patient had required glucocorticoid therapy (which she does not), it would have been essential to start corticosteroids several days before initiating thyroid hormone replacement to prevent precipitating adrenal crisis—but this precaution is unnecessary given her normal adrenal reserve. 1
Common Pitfall to Avoid
Do not empirically start glucocorticoid replacement in patients with hypothyroidism and low baseline cortisol without first performing an ACTH stimulation test. 1, 2