Hepatic bioavailability of serum thyroid hormones in nonthyroidal illness

Stress-induced hormonal alterations

The hormonal alterations observed during critical illness are extremely diverse, being both adaptive and maladaptive. Central and peripheral mechanisms are at play depending on the duration and severity of the insult. Perturbations in the endocrine axes are exacerbated by endogenous mediators and by pharmacologic intervention. The emergence of the concept of the wound, or inflammatory lesion, as an endocrine organ introduces new potential mechanisms of stress physiology and possible interventional strategies. Whether the physiology of the wound can be manipulated to heighten organism resistance to stress either emergently or electively is theoretically appealing and supported by animal models. Currently, however, it remains a subject of ongoing controversy and research.

Euthyroid sick syndrome: an overview

Abnormalities of thyroid hormone concentrations are seen commonly in a wide variety of nonthyroidal illnesses, resulting in low triiodothyronine, total thyroxine, and thyroid stimulating hormone concentrations. These thyroid hormone changes may be mediated in part by cytokines or other inflammatory mediators, acting at the level of the hypothalamus and pituitary, the thyroid gland, and the hepatic deiodinase system, as well as on binding of thyroxine to thyroid binding globulin. The degree of thyroid function disturbance correlates with disease severity and low levels of thyroid hormones predict a poor prognosis in several illnesses. It remains unresolved whether the hormone responses in the euthyroid sick syndrome represent part of an adaptive response, which lowers tissue energy requirements in the face of systemic illness, or a maladaptive response, which induces damaging tissue hypothyroidism. Consequently, the use of thyroid hormone therapy in the euthyroid sick syndrome is controversial. The small number of controlled trials performed to date have shown conflicting results on the cardiovascular effects of triiodothyronine, and none has had the statistical power to address the question of altered mortality. Future trials of therapy should concentrate on patients with severe nonthyroidal illness and a high mortality rate. Meanwhile, better understanding is needed of the impact of the altered thyroid hormone status on tissue function.

Reduced tissue thyroid hormone levels in fatal illness

Patients with severe nonthyroidal illnesses (NTIs) frequently have decreased serum concentrations of triiodothyronine (T3) and less often of thyroxine (T4) without clear evidence of hypothyroidism. To determine whether T3 and T4 levels are also reduced in the tissues, we analyzed autopsy samples from 12 patients dying of NTI and 10 previously healthy individuals dying suddenly from trauma. Mean serum T3, T4, and free T4 index values were lower by 79%, 71%, and 49%, respectively, in the NTI group than in controls, but serum thyrotropin (TSH) values did not differ significantly. Mean T3 concentrations in cerebral cortex, hypothalamus, pituitary, liver, kidney, and lung were lower in the NTI group than in controls by 43% to 76%, but mean values in heart and skeletal muscle did not differ significantly between the groups. The mean liver T4 concentration was 66% lower in the NTI group, but mean T4 concentrations in the cerebral cortex were similar in the two groups. These results indicate that many tissues may be deficient in thyroid hormones in patients with fatal NTI, although the severity of the reduction in thyroid hormone concentrations may vary from one organ to another.

Thyroid hormone and the gut

The gastrointestinal tract interacts actively with the thyroid hormones, T4 and T3. Both T4 and T3 are absorbed well but incompletely from the gut, and many factors affect this absorption. The mechanism of absorption is unknown. It is decreased in most malabsorption conditions, but is increased in the postgastrojejunotomy syndrome. It may involve conjugation to the glucuronide forms (T4G and T3G) in the mucosal cell with subsequent deconjugation prior to appearance in the portal vein blood. Absorption appears to be reduced in the presence of excess T4, and increased in hypothyroidism. The liver takes up a large fraction of the T4 and T3 from its circulation and returns a portion of the portal hormone back to the gut via the bile. There is also direct T4 and T3 secretion into the gut from the mesenteric circulation. Recent studies suggest that the gut plays a major role as a reservoir for the thyroid hormones, especially for T3, and that it may also play a role in the regulation of hormone activity.

Effects of nonthyroidal illness on thyroid function

Despite the absence of thyroid disease, patients with nonthyroidal illness frequently have changes in serum thyroid hormone measurements that may suggest either hypothyroidism or hyperthyroidism. Serum T3 levels are frequently decreased mainly because of a decrease in the rate of T3 production from T4. The free T3 concentration may be either normal or reduced as well. The binding of T4 and T3 by the serum-binding proteins is almost always impaired, resulting in an increase in the dialyzable fraction (free) fraction. This is due to a decrease in the concentration of thyroxine-binding proteins and the presence of circulating inhibitors of binding. If serum T4 concentration remains within the normal range, the free T4 concentration can be increased. However, serum T4 is frequently decreased in patients with chronic and/or severe illness. The decrease in serum T4 in these patients, combined with an increase in the dialyzable fraction, results in normal free T4. In patients who are critically ill, none of the available methods for measurement of free T4 may give results that accurately reflect the euthyroid state. Since T3 is the major active thyroid hormone, it is surprising that patients with decreased serum T3 do not appear hypothyroid. The decrease in serum T3 is probably an adaptive change to nonthyroidal illness, which at least enables the sick patient to conserve protein. The clinical impression of euthyroidism is supported by the finding of a normal serum TSH level in most patients. Although TSH regulation may not be entirely normal in patients with nonthyroidal disease, it is likely that serum TSH will be increased in most sick patients who also have significant thyroid failure. The normal clinical findings in patients with decreased serum T3 may result from an augmentation of those biologic responses associated with the clinical manifestations of the euthyroid state. Several animal models of nonthyroidal disease or starvation show that cells have the ability to modulate some biologic responses to thyroid hormone. Further study should elucidate the mechanisms underlying these changes. This article has emphasized that no single laboratory measurement may reliably predict the thyroid state in patients with nonthyroidal disease. This fact emphasizes the need for careful clinical evaluation of these patients and judicious use of laboratory tests. Because the changes in thyroid hormone metabolism that occur in nonthyroidal disease probably represent adaptive changes to the illness, treatment with L-thyroxine to restore serum thyroid concentrations to the normal range is not indicated.

A longitudinal evaluation of thyroid function in critically ill surgical patients

Thyroid hormone alterations (known as the "sick-euthyroid syndrome") are common following major surgery, but the time course for appearance and recovery from these alterations has not previously been longitudinally studied in a large group of surgical patients. The authors prospectively studied 59 patients undergoing major surgery (coronary artery bypass grafting, pneumonectomy, or subtotal colectomy). Compared with preoperative values, the mean serum T4, T3, free T3, and TSH concentrations decreased significantly (p less than 0.05) following surgery. Serum reverse T3 and T3 resin uptake index increased, while free T4 levels remained unchanged. These changes were seen within 6 hours of surgery and normalized by 1 week after surgery. Although the serum TSH response to TRH was normal before and after surgery in 56 of the 59 patients, the maximal TRH-induced increase in serum TSH and the integrated serum TSH response to TRH were suppressed in the early perioperative period. This postoperative TSH suppression correlated with elevated postoperative plasma dopamine concentrations (r = 0.57, p less than 0.05). Three patients with compensated primary hypothyroidism were detected in the study and represent the first documentation of serial thyroid hormone and TSH levels in hypothyroid patients undergoing major surgery. These patients had similar changes in thyroid hormone values compared with euthyroid patients. The serum TSH response to TRH was suppressed into the normal range in two of these patients on the day following surgery. The authors conclude that the sick-euthyroid syndrome occurs within a few hours of major surgery and remits with convalescence. Postoperative decreases in serum TSH may mask the diagnosis of hypothyroidism. Surgical consultants should be aware of these rapid postoperative changes so that thyroid function tests are properly interpreted in patients who have undergone major surgery.

Suppression of thyrotropin in the low-thyroxine state of severe nonthyroidal illness

In a prospective study, we assessed the role of thyrotropin in the development of the low-thyroxine state that is associated with severe illness. We measured the serum thyrotropin and thyroid hormone concentrations longitudinally in 35 patients with hematopoietic cancer or aplastic anemia who were treated by bone-marrow transplantation. In 19 patients thyroxine declined sharply after bone-marrow transplantation and was associated with a reduction of the serum thyrotropin in the 17 patients tested, often to levels below the normal range. The serum triiodothyronine level, free thyroxine index, and free thyroxine level also declined in these patients. In the patients who recovered, clinical improvement was accompanied by the return of thyrotropin and thyroid hormone concentrations to their pretreatment ranges. These and related findings suggest that the low-thyroxine state of severe illness is the result of several events, one of which is failure of the normal negative-feedback control of the pituitary-thyroid axis due to illness-associated, decreased secretion of thyrotropin. The notion that such patients are "euthyroid" must be questioned, but the possible value of thyroid hormone-replacement therapy in these circumstances remains to be determined.