Endocrinology and Metabolism Clinics of North America
The Nonthyroidal Illness Syndrome
Section snippets
Alterations of thyroid economy with nonthyroidal illness
Thyroid hormone parameters in nonthyroidal illness have been reviewed in detail elsewhere [10]. We provide a brief summary of the changes typically observed.
Starvation and fasting
The fasting state causes a down-regulation in the hypothalamic-pituitary-thyroid axis and hence decreased thyroid hormone levels [14], [15]. It may be difficult to distinguish between the effects on thyroid function of a given systemic illness versus those of the associated absolute or relative starvation because malnutrition is a component of many acute and chronic diseases. The decreased serum T3 in starvation is hypothesized to reflect an attempt by the organism to conserve energy by
Effects of drugs on thyroid economy
Pharmacologic agents administered to patients who have systemic illness may confound the interpretation of thyroid function tests. A complete review of drug effects on the hypothalamic-pituitary-thyroid axis is beyond the scope of this article and has been reviewed previously [72], [73]. The following section highlights the alterations in thyroid function parameters secondary to drugs commonly used in severe systemic illness.
Thyroid hormone treatment during nonthyroidal illness
The commonly held notion that patients who have nonthyroidal illness are euthyroid continues to be debated [1], [5], [91], [92], [93], [94], [95], [96], [97]. The metabolic state in these patients has been deemed to be euthyroid based on generally normal TSH and free T4 measurements. Changes in thyroidal economy may play an adaptive role in times of stress, but consideration has also been given to the possibility that patients who have nonthyroidal illness and low thyroid hormone levels may not
Summary
The evaluation of altered thyroid function parameters in systemic illness and stress remains complex because changes occur at all levels of the hypothalamic-pituitary-thyroid axis. Nonthyroidal illness syndrome is generally characterized by low serum T3, normal free T4 and TSH, and elevated rT3 values. Unique changes in thyroid function parameters are observed in various clinical states, including starvation and fasting, cardiac disease, renal disease, hepatic disease, and infection. Many
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