Diagnosis: Central (Secondary) Hypothyroidism
This patient has central hypothyroidism, characterized by a low free T4 (0.69 ng/ml, just below the reference range of 0.70-1.48) with an inappropriately normal TSH (1.99 micro IU/ml), indicating pituitary or hypothalamic dysfunction rather than primary thyroid disease. 1
Key Diagnostic Features
The hallmark of central hypothyroidism is a low or low-normal free T4 with a normal or low TSH, which distinguishes it from primary hypothyroidism where TSH would be elevated. 1 In this case:
- Free T4 is 0.69 ng/ml (below the lower limit of 0.70 ng/ml)
- TSH is 1.99 micro IU/ml (inappropriately normal given the low T4)
- Free T3 is 2.47 pg/ml (within normal range but in the lower half)
The TSH should be elevated in response to low thyroid hormone, but remains in the normal range, indicating the pituitary is not responding appropriately. 1
Why This Is NOT Primary Hypothyroidism
Primary hypothyroidism requires both an elevated TSH (typically >4.5-10 mIU/L) AND a low free T4. 1 This patient's TSH of 1.99 micro IU/ml is well within the normal reference range (0.35-4.94), making primary hypothyroidism impossible. 1
Subclinical hypothyroidism is defined as elevated TSH with normal free T4 1—the opposite of this patient's presentation.
Required Workup for Central Hypothyroidism
Before initiating thyroid hormone replacement, this patient requires:
- Morning (8 AM) ACTH and cortisol levels or 1 mcg cosyntropin stimulation test 1
- Gonadal hormones: testosterone (in men), FSH, LH 1
- MRI of the sella with pituitary cuts to evaluate for pituitary mass, hypophysitis, or other structural abnormalities 1
Critical caveat: If both adrenal insufficiency and hypothyroidism are present, steroids must always be started BEFORE thyroid hormone replacement to avoid precipitating an adrenal crisis. 1
Additional Considerations
Central hypothyroidism can result from:
- Pituitary adenomas or other sellar masses 1
- Hypophysitis (though this typically presents with headache and more severe hormone deficiencies) 1
- Previous head and neck radiation 2
- Hypothalamic dysfunction 1
The patient is asymptomatic, which is consistent with mild central hypothyroidism, as symptoms may be subtle or absent in early disease. 3
Management Implications
Once central hypothyroidism is confirmed and other pituitary hormone deficiencies are assessed: