Clinical and laboratory assessment of thyroid abnormalities

A randomised, controlled trial to determine the effect of levothyroxine on Standardbred racehorses

BACKGROUND

The use of thyroid supplement is pervasive in athletic horses although its effects on measures of performance are not known.

OBJECTIVES

One purpose of this study was to determine whether supra-physiologic doses of levothyroxine affect the velocities at which blood lactate was greater than 4 mmol/L (V_La4 ) and heart rate was over 150 (V₁₅₀ ) and 200 (V₂₀₀ ) beats per minute respectively. Additionally, a survey of post-race blood samples was also conducted to determine whether high thyroxine concentrations were common in racehorses.

STUDY DESIGN

A randomised, crossover, trial was performed in six healthy Standardbred racehorses.

METHODS

Study 1: T4 was determined in 50 post-race samples from a single Standardbred meet. Study 2: Research horses were trained to fitness and then randomised to one of three treatments: carrier, 0.1 mg/kg thyroxine or 0.25 mg/kg thyroxine for 2 weeks. Horses completed a standardised exercise treadmill test (SET) to fatigue on the last day of treatment. Serum free and total thyroxine and triiodothyronine were determined on the day of SET testing. Blood lactate and ECG data were collected during the SET at 6, 8, 10, 11, and 12 m/s and during recovery. The effect of treatment and SET on heart rate and blood lactate was examined using generalised linear mixed models. Post hoc analysis was adjusted for multiple comparisons using Tukey's Test. Data were expressed as mean ± standard deviation and P < .05 was considered significant.

RESULTS

Study 1: The median T4 value in this population of horses was 2.00 µg/dL (laboratory's normal range 1.5-4.5 µg/dL) and 3 of 50 racehorses had values above the laboratory reference range. Study 2: Levothyroxine at 0.25 mg/kg resulted in higher heart rates during SET (199 ± 30, 223 ± 17 and 239 ± 9 bpm at 6, 8 and 10 m/s respectively) and recovery (144 ± 20 and 119 ± 15 at 5 and 15 min) as compared to placebo (176 ± 18, 203 ± 10 and 219 ± 6 bpm at 6, 8, and10 m/s and 126 ± 5, 102 ± 11 at 5-15 minutes respectively). Three of six horses developed cardiac arrhythmias including atrial fibrillation.

MAIN LIMITATIONS

A relatively small number of animals were used and a SET is not identical to actual racing conditions.

CONCLUSIONS

Supra-physiologic thyroxine supplementation caused a decreased V₂₀₀ during a standard exercise test and may result in cardiac arrhythmias.

Hepatic failure resulting from thyroid storm with normal serum thyroxine and triiodothyronine concentrations

A 63-year-old male presented with productive cough, fever, palpitation, shortness of breath, and mental confusion. Fulminant hepatic failure occurred with high aminotransferase and severe jaundice. Thyroid function tests showed thyroid-stimulating hormone level of 0.7 microU/mL (normal range, < 6.2 microU/mL), with normal serum thyroxine and triiodothyronine levels. He died on post-admission day 5. Autopsy revealed diffuse hyperplasia of the thyroid gland, centrilobular necrosis of the liver, and congestion of the gastrointestinal tract, spleen, and adrenal glands. These pathological findings were compatible with decompensated hyperthyroidism. Thyroid storm may present with normal serum triiodothyronine, thyroxine, and hepatic failure. Such presentation is rare.

Trauma and thyroid-induced tachycardia

Excluding a source of hemorrhage after blunt trauma in a patient who presents with a sustained tachycardia can be challenging. This report is of two trauma patients presenting with undiagnosed thyrotoxicosis. Trauma-triggered thyrotoxicosis is rarely reported in the literature. The confusing presentation, the laboratory analysis, and the response to therapeutic intervention are reviewed.

Observer variation in the clinical assessment of the thyroid gland

In order to evaluate the reliability of clinical assessment of the thyroid gland, two specialists in endocrinology and two younger doctors independently examined 53 patients twice, and assessed whether they had a diffuse goitre, a multinodular goitre, a solitary nodule or a normal gland. In 30% of the patients all four observers were in agreement, whereas in 47% and 23% of the patients, two and three different diagnoses were given, respectively. Inter-observer variation was determined and kappa values between -0.04 and 0.54 were found. Intra-observer variation was smaller, revealing kappa values between 0.44 and 1.00. The present study suggests that clinical assessment of the thyroid gland may lead to misclassification of the type of thyroid disease, and thereby to a less than optimal choice of therapy.

Thyroid crises

In the setting of characteristic features of thyrotoxicosis, the timely diagnosis and aggressive management of thyroid storm should result in a successful outcome. However, severe storm may lead to irreversible cardiovascular collapse, especially in the older patient who may have atypical features of thyrotoxicosis. The fundamental approach is prompt and optimal treatment in the emergency department once the presenting clinical features suggest its presence. Delay in the introduction of therapy while awaiting laboratory confirmation may result in further decompensation and death. The prevention of myxedema coma entails paying special attention to certain high-risk patient groups. These groups include older women with a history of Hashimoto's thyroiditis, or previous irradiation or thyroid surgery for hyperthyroidism. Inform such patients of the symptoms and signs of hypothyroidism, and perform annual thyroid function tests, such as a serum TSH, in order to provide early, adequate treatment once the test becomes positive.

Effective laboratory evaluation of thyroid status

This article focuses on recent developments in thyroid-related laboratory tests, including analytical methods, clinical utility, and limitations of TSH, FT4, T4, FT3/T3, thyroglobulin, and thyroid autoantibodies and the effective use of these tests in the diagnosis of various forms of hypothyroidism or hyperthyroidism, and the management of patients undergoing T4 replacement, T4 suppression, or antithyroid drug therapy.

Anaesthesia for caesarean section in patients with uncontrolled hyperthyroidism

The anaesthetic management of two patients with uncontrolled hyperthyroidism requiring Caesarean section is described. The first patient was diagnosed as hyperthyroid during pregnancy but was resistant to medical management with propylthiouracil (PTU). At 38 weeks' gestation, after optimizing her heart rate with intravenous propranolol, she received an epidural anaesthetic for the Caesarean section. The baby had manifestations of hypothyroidism immediately after birth but later became hyperthyroid. The second patient's hyperthyroidism was well controlled but she stopped her medication one week before delivery. The Caesarean section was performed under general anaesthesia for fetal distress. The baby died in the neonatal period of extreme prematurity. The anaesthetic management of the hyperthyroid parturient is discussed. Preoperative control of hyperthyroidism, bearing in mind pregnancy and time constraints, is essential. The use of PTU, propranolol, iodine, and glucocorticoids is described in the preparation of the parturient. Fetal and neonatal effects of these drugs are discussed.