Thyroid function profile in infants submitted to cardiac surgery with cardiopulmonary bypass

Abstracts

BACKGROUND AND OBJECTIVES: Cardiac surgery with cardiopulmonary bypass (CPB) is related to euthyroid disease or hypothalamic-pituitary-thyroid system depression. Abnormal hemodynamic status induced by CPB is responsible for several endocrine-metabolic changes, triggering complex systemic inflammatory response. This study aimed at evaluating triiodothyronine (T3), tetraiodothyronine (T4) and thyrotrophin (TSH) behavior in infants submitted to cardiac surgery with CPB. METHODS: Participated in this study 15 infants. Blood samples for T3, T4 and TSH evaluation were collected in four moments: M1 - after anesthetic induction; M2 - at surgery completion; M3 - six hours after surgery completion; M4 - 24 hours after M1. To complete this study the following parameters were evaluated: mean blood pressure (MBP), central blood temperature (ºC), tissue oxygenation and acid-base attributes (M1, CPB, M2, M3 and M4). RESULTS: The patients' means aged, weigh, height and body surface were 3.9 months; 4.708 kg; 0.65 m and 0.3 m² respectivety. Plasma T3 (p < 0.0001), T4 (p < 0.0001) and TSH (p = 0.0021) concentrations have significantly varied throughout the study with T3 concentrations progressively decreasing. Lowest T3 and T4 values were coincident with highest Ht and Hb values, discarding hemodilution effects. Highest serum TSH concentrations have shown a possible hypothalamic-pituitary-thyroid system reaction to hypothermia and of massive iodine absorption (topic use of antiseptic solutions). "Low T3 Syndrome" aspects were identified in all studied moments. CONCLUSIONS: There have been changes in serum T3, T4 and TSH concentrations in infants submitted to cardiac surgery with cardiopulmonary bypass.

ANESTHESIA; HORMONES; SURGERY

JUSTIFICATIVA E OBJETIVOS: A cirurgia cardíaca com circulação extracorpórea (CEC) relaciona-se à doença eutireóidea ou à depressão do eixo hipotálamo-hipofisário-tireóideo. O estado hemodinâmico incomum imposto pela CEC é responsável por diversas alterações endócrino-metabólicas, acarretando complexa resposta inflamatória sistêmica. Esta investigação teve como objetivo analisar o comportamento da triiodotironina (T3), tetraiodotironina (T4) e tireotrofina (TSH) em lactentes submetidos à cirurgia cardíaca com CEC. MÉTODO: Foram estudados 15 lactentes. As amostras de sangue para pesquisa de T3, T4 e TSH foram obtidas em 4 momentos designados: M1 - após a indução da anestesia; M2 - após o final da cirurgia; M3 - 6 horas após o final da cirurgia e M4 - 24 horas após o M1. Para complementar esta investigação, foram estudadas as variações dos seguintes parâmetros: pressão arterial média (PAM), temperatura sangüínea central (ºC), atributos da oxigenação tecidual e do equilíbrio ácido-base (M1, CEC, M2, M3 e M4). RESULTADOS: As médias de idade, peso, altura e superfície corpórea dos pacientes foram 3,9 meses; 4,708 kg; 0,65 m e 0,3 m², respectivamente. As concentrações plasmáticas de T3 (p < 0,0001), T4 (p < 0,0001) e TSH (p = 0,0021) variaram significativamente durante o período estudado, sendo que as de T3 declinaram progressivamente. As menores taxas de T3 e T4 coincidiram com as maiores de Ht e Hb, descartando os efeitos da hemodiluição. As maiores concentrações séricas de TSH demonstraram a provável reação do eixo hipotálamo-hipofisário-tireóideo aos efeitos da hipotermia e da absorção maciça pelo iodo (uso tópico de soluções anti-sépticas). Foram identificados aspectos da "Síndrome do T3 baixo" em todos os momentos estudados. CONCLUSÕES: Foram observadas alterações nas concentrações séricas de T3, T4 e TSH em lactentes submetidos à cirurgia cardíaca com circulação extracorpórea.

ANESTESIA; CIRURGIA; HORMÔNIOS

JUSTIFICATIVA Y OBJETIVOS: La cirugía cardíaca con circulación extracorpórea (CEC) relacionase a la enfermedad eutiroidea o a la depresión del eje hipotálamo-hipofisário-tiroideo. El estado hemodinámico incomún impuesto por la CEC es responsable por diversas alteraciones endócrino-metabólicas, acarreando compleja respuesta inflamatoria sistémica. Esta investigación tuvo como objetivo analizar el comportamiento de la triiodotironina (T3), tetraiodotironina (T4) y tireotrofina (TSH) en lactantes sometidos a cirugía cardíaca con CEC. MÉTODO: Fueron estudiados 15 lactantes. Las muestras de sangre para pesquisa de T3, T4 y TSH fueron obtenidas en 4 momentos designados: M1 - después de la inducción de la anestesia; M2 - después del final de la cirugía; M3 - 6 horas después del final de la cirugía y M4 - 24 horas después del M1. Para complementar esta investigación, fueron estudiadas las variaciones de los siguientes parámetros: presión arterial media (PAM), temperatura sanguínea central (ºC), atributos de la oxigenación del tejido y del equilibrio ácido-base (M1, CEC, M2, M3 y M4). RESULTADOS: Las medias de edad, peso, altura y superficie corpórea de los pacientes fueron 3,9 meses; 4,708 kg, 0,65 m y 0,3 m²respectivamente. Las concentraciones plasmáticas de T3 (p < 0,0001), T4 (p < 0,0001) y TSH (p = 0,0021) variaran significativamente durante el período estudiado, siendo que las de T3 declinaran progresivamente. Las menores tasas de T3 y T4 coincidieran con las mayores de Ht y Hb, descartando los efectos de la hemodilución. Las mayores concentraciones séricas de TSH demostraran la probable reacción del eje hipotálamo-hipofisário-tiroideo a los efectos de la hipotermia y de la absorción maciza por el yodo (uso tópico de soluciones anti-sépticas). Fueron identificados aspectos de la "Síndrome del T3 bajo" en todos los momentos estudiados. CONCLUSIONES: Fueron observadas alteraciones en las concentraciones séricas de T3, T4 y TSH en lactantes sometidos a la cirugía cardíaca con circulación extracorpórea.

SCIENTIFIC ARTICLE

Thyroid function profile in infants submitted to cardiac surgery with cardiopulmonary bypass^*

Aspectos de la función tiroidea en lactantes sometidos a cirugía cardíaca con circulación extracorpórea

Paula Maria da Cruz, M.D.^I; Carmem Narvaes Bello, TSA, M.D.^II; Miguel Lorenzo Barbero Marcial, M.D.^III; José Otávio Costa Auler Júnior, TSA, M.D.^IV

^IMestre em Medicina pela Faculdade de Medicina da Universidade de São Paulo (Área de Concentração: Anestesiologia); Médica pertencente ao Corpo Clínico do Hospital São Joaquim da Real e Benemérita Sociedade Portuguesa de Beneficência

^IIDoutora em Medicina pela FMUSP; Médica Assistente do Departamento de Anestesiologia do HCFMUSP

^IIIDiretor da Unidade Cirúrgica de Cardiologia Pediátrica do Instituto do Coração do HCFMUSP

^IVProfessor Titular da Disciplina de Anestesiologia da FMUSP; Diretor da Divisão de Anestesiologia do HCFMUSP

^{Correspondence}

SUMMARY

BACKGROUND AND OBJECTIVES: Cardiac surgery with cardiopulmonary bypass (CPB) is related to euthyroid disease or hypothalamic-pituitary-thyroid system depression. Abnormal hemodynamic status induced by CPB is responsible for several endocrine-metabolic changes, triggering complex systemic inflammatory response. This study aimed at evaluating triiodothyronine (T₃), tetraiodothyronine (T₄) and thyrotrophin (TSH) behavior in infants submitted to cardiac surgery with CPB.

METHODS: Participated in this study 15 infants. Blood samples for T₃, T₄ and TSH evaluation were collected in four moments: M₁ - after anesthetic induction; M₂ - at surgery completion; M₃ - six hours after surgery completion; M₄ - 24 hours after M₁. To complete this study the following parameters were evaluated: mean blood pressure (MBP), central blood temperature (ºC), tissue oxygenation and acid-base attributes (M₁, CPB, M₂, M₃ and M₄).

RESULTS: The patients' means aged, weigh, height and body surface were 3.9 months; 4.708 kg; 0.65 m and 0.3 m² respectivety. Plasma T₃ (p < 0.0001), T₄ (p < 0.0001) and TSH (p = 0.0021) concentrations have significantly varied throughout the study with T₃ concentrations progressively decreasing. Lowest T₃ and T₄ values were coincident with highest Ht and Hb values, discarding hemodilution effects. Highest serum TSH concentrations have shown a possible hypothalamic-pituitary-thyroid system reaction to hypothermia and of massive iodine absorption (topic use of antiseptic solutions). "Low T₃ Syndrome" aspects were identified in all studied moments.

CONCLUSIONS: There have been changes in serum T₃, T₄ and TSH concentrations in infants submitted to cardiac surgery with cardiopulmonary bypass.

Key Words: ANESTHESIA, Pediatric; HORMONES: thyroid; SURGERY, Cardiac

RESUMEN

JUSTIFICATIVA Y OBJETIVOS: La cirugía cardíaca con circulación extracorpórea (CEC) relacionase a la enfermedad eutiroidea o a la depresión del eje hipotálamo-hipofisário-tiroideo. El estado hemodinámico incomún impuesto por la CEC es responsable por diversas alteraciones endócrino-metabólicas, acarreando compleja respuesta inflamatoria sistémica. Esta investigación tuvo como objetivo analizar el comportamiento de la triiodotironina (T₃), tetraiodotironina (T₄) y tireotrofina (TSH) en lactantes sometidos a cirugía cardíaca con CEC.

MÉTODO: Fueron estudiados 15 lactantes. Las muestras de sangre para pesquisa de T₃, T₄ y TSH fueron obtenidas en 4 momentos designados: M₁ - después de la inducción de la anestesia; M₂ - después del final de la cirugía; M₃ - 6 horas después del final de la cirugía y M₄ - 24 horas después del M₁. Para complementar esta investigación, fueron estudiadas las variaciones de los siguientes parámetros: presión arterial media (PAM), temperatura sanguínea central (ºC), atributos de la oxigenación del tejido y del equilibrio ácido-base (M₁, CEC, M₂, M₃ y M₄).

RESULTADOS: Las medias de edad, peso, altura y superficie corpórea de los pacientes fueron 3,9 meses; 4,708 kg, 0,65 m y 0,3 m²respectivamente. Las concentraciones plasmáticas de T₃ (p < 0,0001), T₄ (p < 0,0001) y TSH (p = 0,0021) variaran significativamente durante el período estudiado, siendo que las de T₃ declinaran progresivamente. Las menores tasas de T₃ y T₄ coincidieran con las mayores de Ht y Hb, descartando los efectos de la hemodilución. Las mayores concentraciones séricas de TSH demostraran la probable reacción del eje hipotálamo-hipofisário-tiroideo a los efectos de la hipotermia y de la absorción maciza por el yodo (uso tópico de soluciones anti-sépticas). Fueron identificados aspectos de la "Síndrome del T₃ bajo" en todos los momentos estudiados.

CONCLUSIONES: Fueron observadas alteraciones en las concentraciones séricas de T₃, T₄ y TSH en lactantes sometidos a la cirugía cardíaca con circulación extracorpórea.

INTRODUCTION

Cardiac surgery with cardiopulmonary bypass (CPB) is related to euthyroid disease or hypothalamic-pituitary-thyroid system depression. Abnormal hemodynamic status induced by CPB is responsible for several endocrine-metabolic changes, triggering complex systemic inflammatory response ¹. Euthyroid disease, evidenced by transient thyroidal hormones peripheral metabolism unbalance, is interpreted as an organic defense mechanism aiming at decreasing baseline metabolism, but it may also mean poor prognosis ^2-4. In the lab it is manifested in two ways ⁵:

1. "Low T₃ Syndrome": presence of decreased T₃ serum concentrations and high reverse T₃ concentrations (T₃r) in the presence of normal T₄ and TSH. This phenomenon is caused by lower desiodinase type 1 - 5 activity, which is the enzyme normally responsible for generating T₃ as from T₄ and for the production of diiodothyronines as from T₃r (inactive metabolite). As result, T₃ is not satisfactorily produced while serum T₃r rates are increased. The situation is further worsened by further alternative metabolic pathway action through desiodinase type 1 - 5, which generates T₃r (but not T₃) as from T₄. Serum T₃r rates are then increased by higher production and inadequate clearance.

2. "Low T₃ and T₄ Syndrome": more severe, caused by deficient TSH stimulation and followed by the decrease or inefficacy of thyroid hormone carrying proteins. In this case, serum T₃r concentrations are virtually not changed because type 1 - 5 desiodinase (alternative pathway) does not find enough substrate, which is T₄.

Euthyroid disease has already been documented in adults submitted to cardiac surgery with CPB; however pediatric patients have uncertain behavior due to immaturity ⁶.

This study aimed at evaluating T₃, T₄ and TSH behavior in infants submitted to cardiac surgery with CPB.

METHODS

After the Ethics Committee approval, 15 infants, physical status ASA III, IV and V, submitted to cardiac surgery for cyanotic and non-cyanotic disease correction were prospectively evaluated. Exclusion criteria were children with congenital thyroidal metabolism disorders, hypothalamic-pituitary-thyroid system disorders, having received iodine contrast (cardiac catheterization) or with parents with thyroid dysfunction. Blood samples for T₃, T₄ and TSH evaluation were collected in four moments: M₁ - after anesthetic induction; M₂ - at surgery completion; M₃ - 6 hours after surgery completion; M₄ - 24 hours after M₁. The following parameters were also evaluated: in moments M₁, CPB (maximum hypothermia determined by the surgical team), M₂, M₃ and M₄, the behavior of the following variables: mean blood pressure (MBP), central blood temperature (esophageal thermometer, ºC), hematocrit (Ht), hemoglobin (Hb), oxygen partial pressure (PaO₂), oxygen arterial saturation (SpO₂), blood pH (pH), excess bases (EB) and bicarbonate ion (HCO₃^-).

General anesthesia was induced with fentanyl, midazolam and pancuronium ⁷. Antiseptic technique consisted of extensive topic application of anti-germ solution (1% active iodine, 100 mL), followed by refined ethyl alcohol and Povidine^® (10% active iodine). Children were submitted to perfusion with pediatric membrane oxygenator under assisted hypothermia (28 to 20 ºC) and non-pulsatile blood flow generated by the CPB pump roller system (1.5 L.min^-1). Perfusate (600 to 700 mL ) aimed at maintaining hematocrit above 30% through the administration of blood products ⁸. T₃ and T₄ were dosed by the radioimmunoassay method and TSH was dosed by the immunoradiometric method.

Normal values: T₃ - 0.8 to 2 ng.mL ^-1; T₄ - 4.5 to 12.5 µg.dL ^-1; TSH - 0.3 to 4.5 mUl.mL ^-1.

Means along time were evaluated by profile analysis and quantitative variables by Dunnet and Bonferroni's test, considering significant p < 0.05.

RESULTS

T₃, T₄ and TSH concentrations varied significantly along time (p < 0.05). Highest T₃ (although slightly below normal reference values) and T₄ means were obtained in M₁. Lowest T₃ and T₄ means were found in M₄. Highest TSH concentrations were obtained in M₂ and lowest TSH concentrations were obtained in M₃. Numeric results are shown in table I, while hormonal concentrations along time are shown in figure 1, 2 and 3.

Brazilian Journal of Anesthesiology, 2004; 54: 3: 325 - 334

Thyroid function profile in infants submitted to cardiac surgery with cardiopulmonary bypass

Paula Maria da Cruz; Carmem Narvaes Bello; Miguel Lorenzo Barbero Marcial; José Otávio Costa Auler Júnior

Brazilian Journal of Anesthesiology, 2004; 54: 3: 325 - 334

Thyroid function profile in infants submitted to cardiac surgery with cardiopulmonary bypass

Paula Maria da Cruz; Carmem Narvaes Bello; Miguel Lorenzo Barbero Marcial; José Otávio Costa Auler Júnior

Brazilian Journal of Anesthesiology, 2004; 54: 3: 325 - 334

Thyroid function profile in infants submitted to cardiac surgery with cardiopulmonary bypass

Paula Maria da Cruz; Carmem Narvaes Bello; Miguel Lorenzo Barbero Marcial; José Otávio Costa Auler Júnior

Brazilian Journal of Anesthesiology, 2004; 54: 3: 325 - 334

Thyroid function profile in infants submitted to cardiac surgery with cardiopulmonary bypass

Paula Maria da Cruz; Carmem Narvaes Bello; Miguel Lorenzo Barbero Marcial; José Otávio Costa Auler Júnior

All other parameters have also varied significantly along time (p < 0.05), except for Ht and Hb (Table II). Lowest MBP, central blood temperature, Ht, Hb, pH, EB and HCO₃^- means were obtained at CPB. Highest Ht and Hb concentrations were found in M₄.

Brazilian Journal of Anesthesiology, 2004; 54: 3: 325 - 334

Thyroid function profile in infants submitted to cardiac surgery with cardiopulmonary bypass

Paula Maria da Cruz; Carmem Narvaes Bello; Miguel Lorenzo Barbero Marcial; José Otávio Costa Auler Júnior

DISCUSSION

According to hormonal means obtained, it is possible to consider that early on (M₁) there were already lab aspects of "Low T₃ Syndrome", and several factors may have contributed for such:

a) Malnutrition - congenital heart diseases in general course with significant caloric deprivation originated from inadequate diet, nutrients malabsorption or high metabolic consumption ⁹. Hypercatabolic state, when present, is the result of excessive catecholamines production and not of thyroid hormones hyperactivity. Malnutrition directly affects T₄ conversion into T₃ and, in the presence of liver failure, there are changes in T₃ and T₄ transport due to the lack of plasma carrying proteins TBG (Thyroid-Binding Globulin) and TBPA (Thyroid-Binding Pre-Albumin) ^10,11. Thyroid hormonal activity depression is also common ¹²;

b) Preoperative fasting - exaggerated infant fasting, or worse, in infants with decreased nutritional reserves, promotes circulating fatty acids release which interferes with thyroid hormones transportation by proteins. Adults submitted to 24-hour food suppression have developed "Euthyroid Sick Syndrome", characterized by lower tissue T₄ conversion with progressive generation of T₃r. Oral glucose normalizes serum T₃¹³;

c) Preoperative medication - some drugs, such as furosemide, directly affect biochemical binding of T₃ and T₄ to TBG, TBPA and albumin by the competition mechanism ¹⁴.

Highest serum TSH concentrations (although within normal limits) were obtained in M₂, suggesting that there has been more pituitary stimulation at cardiac surgery completion. However, lab aspects of "Low T₃ Syndrome" still remained.

Topic application of iodinated products (anti-germ and iodine tincture) in children results in massive iodine absorption by the skin not necessarily with previous integrity disruption. Contamination is proven by increased serum and urinary concentrations of this ion ^15-17.

Metabolism is affected in two ways:

a) Lower hormone synthesis by the thyroid (Wolf-Chaikoff effect) because of thyroid-peroxidase inhibition and consequent blockade of organification (or iodination) of thyroglobulin (substance generator of thyroid hormones in glandular parenchyma);

b) Lower peripheral conversion of T₄ into T₃ (euthyroid disease) - since iodine excretion is exclusively achieved by renal clearance, this type of intoxication, in addition to impairing neonate and infant renal function (whose glomerular filtration rate is already physiologically decreased), may lead to prolonged thyroid hypofunction ¹⁸. Primary transient hypothyroidism is configured and characterized by significant plasma TSH increase together with decrease in T₃, T₄ and their free fractions ¹².

Another aspect to be considered is the interference of drugs.

Steroids, used to attenuate CPB-induced inflammatory response, directly reach three metabolic sites: biochemical binding of hormones to carrying proteins (increased free hormones), tissue T₄ conversion, and pituitary TSH secretion decrease, in this case not followed by increased T₃ and T₄, but by the opposite ^12,13.

Dopamine depresses pituitary activity leading to lower TSH production even after stimulation with exogenous TRH (thyrotrophin release factor), with lab installation of secondary or transient pituitary hypothyroidism ¹².

Heparin releases lipases and, as a consequence, triglycerides hydrolysis into non-esterified free fatty acids, impairing the binding of hormones to plasma proteins ¹⁹. Heparin also prevents the binding of hormones to specific protein sites. Increased blood fatty-acids due to its use, however, seems to be of little relevance in neonate and infant cardiac surgeries ²⁰.

Increased free hormones (T₃F and T₄F) during CPB ²¹ have been attributed to the administration of sodium bicarbonate which, like heparin, changes the biochemical affinity of hormones to circulating proteins. Possible prolonged metabolic acidosis may also affect intracellular conversion of T₄ into T₃²².

Systemic inflammatory response, undesirable event inherent to CPB, interferes with thyroid hormones secretion through the release of immune system chemical mediators. The presence of cytokines during pediatric surgeries has been reviewed by Saatvedt et al. ²³ who have observed persistent reverse correlation between serum IL-6 and IL-3 rates. IL-6 and tumor necrosis factor, released during CPB-induced hypoperfusion, possibly due to intestinal bacterial translocation, decrease T₃ generation as from T₄ and decrease serum TSH ^24,25 (depressing effect on pituitary).

Non-pulsatile blood flow, directly responsible for transient anterior pituitary secreting hypofunction, also affects the hypothalamic-pituitary-thyroid system, decreasing the amount of serum cortisol. Exogenous TSH administration does not increase TSH during CPB, thus establishing one more factor contributing for secondary transient hypothyroidism ^26,27.

T₃/Ht ratio has allowed for the investigation of hemodilution effects, but lowest T₃ concentrations were paradoxically found after CPB ²⁸. We have considered a possible influence of inadvertent thyroid hormones administration in the perfusate through blood transfusion, but this was considered irrelevant due to hemodilution ^29,30 (half-lives of T₃, T₄ and TSH present in one red-cell concentrate unit are respectively: 1 to 2 days; 8 days and 54 minutes). Proteins scavenging observed in adult surgeries (with the use of Cell Saver), however, has been substantial and also seems to explain the increase in T₄F after CPB in pediatric surgeries ³¹.

Hypothermia activates the hypothalamic-pituitary-thyroid system promoting sympathetic nervous system hyper-reactivity with the release of catecholamines in the plasma ³². Experimental studies in rats have shown increased plasma TSH 30 minutes after exposure to cold, due to intensive pituitary stimulation with higher TRH production ³³. TRH secretion in these conditions is independent of the amount of TRH previously present in the hypothalamus during normothermal situations ^34,35. Fundamental thyroid metabolic changes in neonates are based on abrupt fetal exposure to the extra-uterine environment, with sudden circulating TSH increase three hours after birth. Chronic exposure to cold is also related to increased T₃ and T₄^12,13.

A comparative study with children (1.5 to 13 months) submitted to CPB under normothermia or hypothermia (±18 ºC) has shown major TSH increase in the second group, suggesting that this phenomenon is not restricted to birth, but is a consequence of anterior hypothalamus pre-optic area cooling ³⁶.

The interpretation of M₂ results shows lower T₃ and T₄ rates together with increased TSH, and it should be highlighted that the effects of acute contamination with iodine and of hypothermia were possibly preponderant. This way, in this study, it was observed that neuraxis was not depressed, suggesting that it has actively responded to these factors.

Progressive T₃ and T₄ decrease would not be related to hemodilution effects, since highest Ht and Hb means were found in M₄. Plasma THS decrease in M₃ seems to be related to the exposure to hormonal secretion depressing agents, such as dopamine, and/or the release of inflammatory response substances (cytokines). However, even in these two situations, there were lab evidences of "Low T₃ Syndrome".

In our study, cardiac surgery with CPB for congenital heart disease correction in infants has promoted significant changes in T₃, T₄ and TSH behavior during the studied period. This way, there are several etiologies promoting thyroid metabolic changes during pediatric cardiac surgeries. The major cause of intraoperative primary hypothyroidism, surely promoted by acute and massive iodine absorption by the skin as a consequence of the use of anti-septics ^16,37, could have been totally abolished since there are other non-iodinated compounds that are also effective. Cases of reoperation in a short period of time may mean further and even more severe contamination. Excess iodine in the body may lead to prolonged thyroid hypofunction state called Wolff-Chalkoff effect ^12-14 (or thyroid peroxidase block), a long time described in adults. This means that during this period, excessive iodine is not incorporated to thyroglobulin molecule present in glandular parenchymal colloidal substance. This organic phenomenon prevents the disproportional production of circulating T₃ and T₄ and consequent systemic metabolic unbalance (hyperthyroidism). Effects of this unbalance in infants are not known, but it is known that it persists while all uptaken and retained iodine in the thyroid is not excreted. Initial result is decreased glandular T₃ and T₄ production with simultaneous serum TSH increase ^13,17.

Some studies have shown that hypothermia is an independent factor promoting, per se, expressive pituitary response with increased circulating TSH ^32-34. In this case, this is an additional effect similar to iodine contamination.

There are however other agents during pediatric cardiac surgeries with CPB, which may simultaneously promote transient secondary hypothyroidism (that is, pituitary response depression) ^17,29,30. Among them, few may be abolished or at least attenuated, since they are inherent to intraoperative and anesthetic techniques, such as the use of drugs as steroids, dopamine, heparin, furosemide, sodium bicarbonate, non-pulsatile blood flow and hemodilution ^{1,8,12,14,19,22,24,27}. One has to consider, however, that in normal physiological conditions, pituitary responds to massive iodine absorption (with increased TSH production), provided there is no situation maintaining it hypofunctioning, such as those intervening during the perioperative period.

The conclusion, then, is that during infant cardiac surgery, thyroid function is the result of all effects acting sometimes on the direct production of hormones by the gland and increasing circulating TSH (primary hypothyroidism), sometimes on the hypothalamic-pituitary-thyroid axis decreasing TSH (secondary hypothyroidism) ²⁰, and sometimes on peripheral thyroid hormones metabolism. These manifestations, if transient, determine the presence of euthyroid disease ^13,31, characterized by reversible but sometimes prolonged thyroid function disorders with extremely relevant postoperative effects ^2-5. In this study it was not possible to investigate the real magnitude of pituitary response to exogenous iodine contamination, although increased serum TSH has prevailed.

REFERENCES

Submitted for publication June 10, 2003

Accepted for publication October 1, 2003

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*

Received from Instituto do Coração da Faculdade de Medicina da Universidade de São Paulo, SP

Publication Dates

• Publication in this collection
  02 July 2004
• Date of issue
  June 2004

History

• Accepted
  01 Oct 2003
• Received
  10 June 2003