Nonthyroidal Illness (NTI) in the PICU

Question 1

What is another name for Nonthyroidal Illness (NTI)?

Answer 1

Euthyroid sick syndrome or low-T3 syndrome.

Question 2

What characterizes NTI biochemically?

Answer 2

Decrease in serum T3, increase in rT3, with normal or slightly decreased FT4 and TSH levels.

Question 3

Why does T3 decrease in NTI?

Answer 3

Reduced activity of type I 5′-monodeiodinase in peripheral tissues, leading to decreased conversion of T4 to T3.

Question 4

What happens to rT3 levels in NTI?

Answer 4

They increase due to impaired clearance, not increased production.

Question 5

How does the severity of illness relate to T3 levels in NTI?

Answer 5

Greater severity correlates with more significant declines in T3, increased rT3, and reduced T3/rT3 ratio.

Question 6

What is the typical TSH level in NTI?

Answer 6

Low-to-normal, often losing circadian rhythm and showing blunted response to TRH.

Question 7

What is the usual level of FT4 in NTI?

Answer 7

FT4 levels are typically normal or slightly decreased, even in severe cases.

Question 8

What happens to pituitary hormones during prolonged illness in NTI?

Answer 8

Suppression of TSH, growth hormone, gonadotropins, and ACTH, suggesting hypothalamic dysfunction.

Question 9

What is observed in the recovery phase of NTI regarding TSH?

Answer 9

TSH may rise slightly above normal, indicating possible recovery of pituitary function.

Question 10

Is thyroid hormone replacement recommended in NTI?

Answer 10

No, unless there is clear clinical and laboratory evidence of hypothyroidism.

Question 11

How can NTI be distinguished from primary hypothyroidism?

Answer 11

Primary hypothyroidism shows TSH >10 mU/L with decreased T4 and T3 levels, while NTI does not.

Question 12

Why is distinguishing between central hypothyroidism and NTI challenging?

Answer 12

Both conditions may present with low-to-normal TSH and normal-to-low FT4 levels.

Question 13

Why can free T4 levels be unreliable in critically ill patients?

Answer 13

Altered protein binding and illness effects can affect the reliability of FT4 assays.

Question 14

What diagnostic tool is essential to differentiate NTI from true hypothyroidism?

Answer 14

Thyroid function panel.

Question 15

What TSH level indicates primary hypothyroidism?

Answer 15

TSH >10 mU/L with decreased T4 and T3 levels.

Question 16

What is the recommended treatment threshold for T4 in NTI?

Answer 16

Consider T4 therapy if FT4 is <0.4 ng/dL with normal TSH, suggesting central hypothyroidism.

Question 17

Should T3 and T4 be routinely administered in NTI?

Answer 17

No, due to risks of increased metabolic rate and catabolism unless overt hypothyroidism is confirmed.

Question 18

What are the primary mechanisms contributing to NTI?

Answer 18

Decreased conversion of T4 to T3 and impaired clearance of rT3.

Question 19

What happens to TSH rhythm in NTI?

Answer 19

Circadian rhythm is lost, and response to TRH is blunted.

Question 20

What is the role of type I 5′-monodeiodinase in NTI?

Answer 20

It is reduced, leading to decreased T3 production and increased rT3.

Question 21

How does illness severity affect the T3/rT3 ratio?

Answer 21

Severe illness leads to a reduced T3/rT3 ratio.

Question 22

What pituitary dysfunctions are observed in prolonged NTI?

Answer 22

Suppression of TSH, ACTH, growth hormone, and gonadotropins.

Question 23

What is a key controversy in NTI management?

Answer 23

Whether or not thyroid hormone replacement should be initiated without confirmed hypothyroidism.

Question 24

What are potential risks of thyroid hormone replacement in NTI?

Answer 24

Increased metabolic rate and catabolism, worsening the patient’s condition.

Question 25

How can FT4 levels guide treatment in NTI?

Answer 25

If FT4 is <0.4 ng/dL with normal TSH, central hypothyroidism should be considered, and treatment may be indicated.