Causes of Low T3 Levels

Primary Causes

Glucocorticoids, dopamine, and dobutamine can suppress thyroid axis function, according to the American College of Clinical Endocrinologists 1
Hypophysitis causes low T4 with low/normal TSH, and subsequently low T3, as reported by the Society for Immunotherapy of Cancer 2
Central hypothyroidism from any cause produces low-normal free T4 rather than high-normal, distinguishing it from subclinical hyperthyroidism, as noted by the American Association of Clinical Endocrinologists 1
Primary hypothyroidism initially presents with high TSH and low T4, but severe cases eventually show low T3 as well, with markedly elevated TSH (typically >10 mIU/L), according to the American College of Physicians 4

Low T3 with low T4 and low/normal TSH is suggestive of severe nonthyroidal illness or central hypothyroidism, as reported by the Society for Immunotherapy of Cancer 2
Low T3 with low T4 and high TSH is indicative of primary hypothyroidism, according to the American College of Physicians 4

Confirmed central hypothyroidism from hypophysitis or other pituitary disease requires physiologic thyroid hormone replacement, but always start steroids first if concurrent adrenal insufficiency exists to avoid precipitating adrenal crisis, as recommended by the Society for Immunotherapy of Cancer 2