Treatment Recommendation for Primary Hypothyroidism with Normal Adrenal Function
Initiate levothyroxine replacement therapy for the primary hypothyroidism; the normal ACTH stimulation test definitively excludes adrenal insufficiency and no glucocorticoid therapy is indicated.
Interpretation of the ACTH Stimulation Test
The patient's cosyntropin stimulation test results are completely normal and rule out adrenal insufficiency:
- Baseline cortisol of 194 nmol/L (approximately 7 μg/dL), rising to 343 nmol/L at 30 minutes and 545 nmol/L at 60 minutes 1
- A peak cortisol >500-550 nmol/L (>18-20 μg/dL) definitively excludes adrenal insufficiency 1
- This patient's peak of 545 nmol/L meets the threshold for normal adrenal function 1
The hypotension (BP 96/68 mm Hg) is not due to adrenal insufficiency but is likely related to the untreated hypothyroidism itself, as hypothyroidism can cause decreased cardiac output and relative hypotension.
Treatment Protocol: Levothyroxine Monotherapy
Start levothyroxine replacement immediately without any glucocorticoid coverage:
- For a 23-year-old woman without cardiac disease, the full replacement dose is approximately 1.6 mcg/kg/day 2
- Administer as a single daily dose on an empty stomach, one-half to one hour before breakfast with a full glass of water 2
- Titrate dosage by 12.5 to 25 mcg increments every 4 to 6 weeks based on serum TSH until the patient is clinically euthyroid 2
- The peak therapeutic effect may not be attained for 4 to 6 weeks 2
Critical Safety Point: No Glucocorticoid Needed
Do not start glucocorticoids before thyroid hormone in this patient because:
- The normal ACTH stimulation test confirms intact adrenal function 1
- The traditional teaching to "always start steroids before thyroid hormone" applies only to patients with confirmed adrenal insufficiency 1
- Starting unnecessary glucocorticoids would expose the patient to iatrogenic Cushing's syndrome without benefit 3
The concern about precipitating adrenal crisis when starting thyroid hormone applies exclusively to patients with documented secondary adrenal insufficiency or hypopituitarism, where glucocorticoid replacement must be initiated several days before thyroid hormone 1. This patient has normal adrenal function.
Monitoring and Follow-Up
- Recheck TSH and free T4 in 4-6 weeks after starting levothyroxine 2
- Target TSH within the normal reference range for primary hypothyroidism 2
- Monitor blood pressure; expect improvement as thyroid function normalizes
- No need for repeat cortisol or ACTH testing unless new symptoms of adrenal insufficiency develop 1
Common Pitfall to Avoid
Do not misinterpret the baseline cortisol of 194 nmol/L as "low" and reflexively start steroids 1. The baseline value is indeterminate on its own, which is precisely why the ACTH stimulation test was performed 1. The robust response to cosyntropin (rising to 545 nmol/L) proves the adrenal glands function normally 1.