The relationship between serum triiodothyronine and thyrotropin during systemic illness

Area of Science:

• Endocrinology
• Metabolic Disorders
• Critical Care Medicine

Background:

• Severe systemic illness can suppress thyroid-stimulating hormone (TSH) response despite low triiodothyronine (T3).
• Stress inhibition of TSH secretion is a known factor in critical illness.

Purpose of the Study:

• To investigate TSH and thyroid hormone dynamics during recovery from severe illness.
• To examine the pituitary's response to thyroid hormone levels in critical illness and hypothermia.

Main Methods:

• Measurement of TSH, total T3, free T3, total T4, and free T4 in patients during recovery from severe illness.
• Analysis of hormone levels in severely ill patients with hypothermia unrelated to cold exposure.

Main Results:

• TSH levels significantly increased during recovery when T3 was rising but still subnormal (6.5 µU/ml vs. normal 2.5 µU/ml).
• Elevated TSH during recovery was negatively correlated with total and free T3, and total T4, but not free T4.
• Severely ill patients with hypothermia showed elevated TSH (5.6 µU/ml) and lower T3 levels compared to other ill patients.

Conclusions:

• The TSH response during recovery from severe illness and in non-cold-induced hypothermia resembles primary hypothyroidism.
• This suggests a pituitary response to perceived thyroid hormone deficiency in these conditions.