What are the reasons for changes in the thyroid hormones during illness?

A 38-year-old female with no previous history of any thyroid problem is admitted in the ICU with septic shock. Avoid doing TFT in a sick patient unless indicated. Patient is on ventilator for past 3 days and has thyroid profile as below: • T3–66 ng/dL (75–200 ng/dL) • T4–3 μg/dL (4.5–11.5 μg/dL) • TSH 0.2 μIU/L (0.45–4.5 μIU/L) A consultation is sent to endocrinology department for treatment of probable hypothyroidism? What is the interpretation of these results? The patient’s differential diagnosis could be: • Secondary hypothyroidism • Sick euthyroid syndrome or nonthyroid illness syndrome (NTIS) The diagnosis of sick euthyroid syndrome is kept as the first possibility in this case because she has no prior history of thyroid problem and has no history of being treated for hypothyroidism in the past. Prior history of deranged TFT can be helpful in reaching a diagnosis. The secondary hypothyroidism is also kept as a possibility so to evaluate for any other pituitary hormone deficiency a random serum cortisol is done along with prolactin, FSH and LH. There is no prior history of pituitary problem in this patient. The FT4 and FT3 are also ordered in this patient. The patient’s FT4 is 1 ng/L (0.8–2 ng/L) and FT3 is 2 pg/mL (2.3–4.2 pg/mL). The serum cortisol is elevated and it is 52 μg/dL because of the stress state and during severe stress serum cortisol can be more than 50 μg/dL and it should be at least more than 25 μg/dL and prolactin is also in the high normal range (36 ng/mL) which is also probably a stress response. Rest of the pituitary hormones are in the normal range. The patient was normally menstruating till the last month so it was also an indirect evidence that the gonadotropic axis is intact. The patient improves over a period of time in the next 2 weeks and thyroid profile is repeated after 2 weeks where it shows the following results: