Can inflammatory cytokines lower active T3 even when TSH is normal?

Based on the patient's presentation and the clinical evidence, we can evaluate the relationship between inflammation, thyroid hormone conversion, and TSH levels:

• Impact of Inflammation on Deiodinase Activity: Pro-inflammatory cytokines (such as IL-6, TNF-alpha, and IL-1beta) directly suppress the activity of peripheral deiodinase enzymes. Specifically, they downregulate the transcription and activity of type 1 (D1) and type 2 (D2) deiodinases, which convert thyroxine (T4) to the active triiodothyronine (T3). Concurrently, these cytokines upregulate type 3 deiodinase (D3), which inactivates thyroid hormones by converting T4 to reverse T3 (rT3).
• Active T3 Availability: Because peripheral conversion by D1 and D2 is the primary source of circulating active T3, the inhibition of these enzymes—combined with the upregulation of D3—directly reduces the availability of active T3. This can lead to a state of localized or systemic tissue-level hypothyroidism.
• TSH Normalcy as an Incomplete Indicator: During systemic inflammation, the pituitary and hypothalamus do not always respond to low peripheral T3 with an increase in TSH. This is due to inflammation-induced alterations in the central feedback loop, such as localized upregulation of D2 in the hypothalamus, which maintains local T3 levels and suppresses TSH secretion. Consequently, serum TSH can remain within normal limits despite a significant deficiency in peripheral active T3.

Bottom line: Inflammatory cytokine signaling can directly impair the peripheral conversion of T4 to active T3 by inhibiting D1 and D2 deiodinases and upregulating D3. This mechanism can significantly reduce active T3 levels and cause tissue-level hypothyroidism, even while serum TSH levels remain entirely within the normal reference range.