Diagnostic Approach to Low TSH with Normal Thyroid Hormones

Definition

Subclinical hyperthyroidism is defined by a serum TSH < 0.45 mIU/L while free T4 and free T3 remain within their reference ranges, after excluding non‑thyroidal causes such as certain medications, pregnancy, severe illness, or recent treatment for overt hyperthyroidism. 1

Medications – Dopamine, high‑dose glucocorticoids, and dobutamine can suppress TSH and must be ruled out before diagnosing subclinical hyperthyroidism. 1
Pregnancy – Physiologic suppression of TSH occurs in the first trimester and should not be misinterpreted as thyroid disease. 1
Severe non‑thyroidal illness – Low or subnormal TSH is common in critical illness; however, an undetectable TSH (< 0.01 mIU/L) is rare unless the patient is receiving glucocorticoids or dopamine. 1
Recent treatment for overt hyperthyroidism – Pituitary recovery of TSH may be delayed after therapy, leading to transient low TSH values. 1
Central hypothyroidism – Pituitary or hypothalamic failure produces low TSH, but free T4 typically lies in the lower‑normal range rather than the upper‑normal range seen in true subclinical hyperthyroidism. 1

In non‑thyroidal illness with low TSH, free T4 is almost invariably in the lower portion of the normal reference interval, whereas in genuine subclinical hyperthyroidism it tends to occupy the upper‑normal range. This distinction helps avoid misclassification. 1

Low TSH values should not be automatically attributed to primary thyroid disease; a systematic evaluation to exclude medications, systemic illness, and physiologic states is essential before confirming subclinical hyperthyroidism. 1