Differential Diagnoses for Hypothyroidism
When evaluating a patient presenting with symptoms and laboratory findings suggestive of hypothyroidism, three key differential diagnoses to consider are: (1) Central (secondary/tertiary) hypothyroidism, (2) Nonthyroidal illness syndrome, and (3) Primary adrenal insufficiency (Addison's disease).
1. Central Hypothyroidism
Pertinent Positives:
- Low or inappropriately normal TSH despite low free T4 (this is the critical distinguishing feature) 1
- History of pituitary/hypothalamic tumors (craniopharyngiomas), inflammatory conditions (lymphocytic or granulomatous hypophysitis), infiltrative diseases, hemorrhagic necrosis (Sheehan's syndrome), or prior pituitary/hypothalamic surgery or radiation 1
- Other pituitary hormone deficiencies may be present (growth hormone, ACTH, gonadotropins, prolactin)
- Visual field defects or headaches if mass lesion present
Pertinent Negatives:
- TSH is NOT elevated (unlike primary hypothyroidism where TSH is high) 1
- Anti-thyroid antibodies (TPOAb, TgAb) are typically negative
- No history of thyroid surgery, radioiodine treatment, or thyroid-directed external beam radiation
- No goiter present
2. Nonthyroidal Illness Syndrome (Sick Euthyroid Syndrome)
Pertinent Positives:
- Acute severe systemic illness or hospitalization 1
- Low T3 and/or low T4 levels
- TSH may be low, normal, or mildly elevated during recovery phase
- Critical illness, sepsis, major surgery, or severe medical conditions present
Pertinent Negatives:
- No true thyroid disease - thyroid function normalizes with recovery from systemic illness
- Anti-thyroid antibodies typically negative
- No prior history of thyroid dysfunction
- TSH measurements in hospitalized patients should only be done if there is specific suspicion for thyroid dysfunction to avoid this diagnostic pitfall 1
3. Primary Adrenal Insufficiency (Addison's Disease)
Pertinent Positives:
- Elevated TSH that may normalize after glucocorticoid replacement 1
- Symptoms of adrenal insufficiency: hyperpigmentation, hypotension, hyponatremia, hyperkalemia, weight loss, fatigue
- May coexist with autoimmune thyroiditis as part of Type 2 Multiple Autoimmune Endocrinopathy (Schmidt's syndrome): Addison's disease + autoimmune thyroiditis + type 1 diabetes 1
- Positive anti-adrenal antibodies, low cortisol, elevated ACTH
Pertinent Negatives:
- The diagnosis of subclinical hypothyroidism should be deferred until after glucocorticoid therapy has been instituted because TSH elevation may be spurious and normalize with treatment 1
- If TSH remains elevated after adequate glucocorticoid replacement, then true primary hypothyroidism is confirmed
Critical Clinical Pitfall
The most important pitfall is failing to recognize central hypothyroidism because clinicians reflexively expect elevated TSH in hypothyroidism. In central hypothyroidism, TSH may be low, normal, or only mildly elevated despite low free T4 1. Always measure free T4 in addition to TSH when evaluating suspected hypothyroidism 1.
Similarly, never diagnose hypothyroidism in a patient with untreated adrenal insufficiency - treat the adrenal insufficiency first, then reassess thyroid function 1. This prevents unnecessary thyroid hormone replacement and avoids precipitating adrenal crisis.