Laboratory Workup for Suspected Central Hypothyroidism

Diagnostic Approach

The initial laboratory workup for suspected central hypothyroidism should include both thyroid-stimulating hormone (TSH) and free thyroxine (FT4) measurements, as central hypothyroidism is characterized by low FT4 with inappropriately low or normal TSH levels, according to the Journal of Clinical Oncology and Journal for ImmunoTherapy of Cancer 1, 2
Measure both TSH and FT4 simultaneously, as measuring TSH alone will miss central hypothyroidism, as recommended by the Journal of Clinical Oncology and Journal for ImmunoTherapy of Cancer 1, 2
Low TSH with a low FT4 is consistent with central hypothyroidism and requires further evaluation, as stated by the Journal of Clinical Oncology and Journal for ImmunoTherapy of Cancer 1, 2, 3
Normal or even mildly elevated TSH with low FT4 can also indicate central hypothyroidism, due to biologically inactive TSH molecules, according to JAMA 4

Proposed confirmation criteria for hypophysitis-induced central hypothyroidism include: ≥1 pituitary hormone deficiency (TSH or ACTH deficiency required) combined with an MRI abnormality, or ≥2 pituitary hormone deficiencies with headache and other symptoms, as suggested by the Journal for ImmunoTherapy of Cancer 2
Clinical suspicion should increase when routine thyroid function testing shows low TSH with low free T4 in patients with symptoms like headache and fatigue, according to the Journal for ImmunoTherapy of Cancer 2, 3

If both adrenal insufficiency and hypothyroidism are present, steroids should always be started prior to thyroid hormone replacement to avoid precipitating an adrenal crisis, as recommended by the Journal for ImmunoTherapy of Cancer 2, 3
If there is uncertainty about whether primary or central hypothyroidism is present, hydrocortisone should be given before thyroid hormone is initiated, according to the Journal of Clinical Oncology 1