Hyperthyroidism is characterized by excessive secretion of thyroxine (T4) and triiodothyronine (T3), leading to a hypermetabolic state. Thyroid dysfunction is one of the most commonly encountered endocrine disorders, affecting about 300 million people worldwide, and nearly half of them are presumed to be unaware of their condition. Hyperthyroidism can have a significant impact on public health and may even shorten the lifespan of individuals of any age. Worldwide, the prevalence of hyperthyroidism in women ranges from 0.5% to 2% and is about ten times more common in women than in men in iodine-replete communities. In Bangladesh, there is no exact data on the prevalence of hyperthyroidism. However, a community-based study conducted among the people of Khulna district reported the incidence of hyperthyroidism to be 0.86% and subclinical hyperthyroidism to be 0.65%. Different studies suggest that the prevalence of hyperthyroidism is increasing gradually.
The cardiovascular system plays a compensatory role in meeting the increased metabolic demand associated with hyperthyroidism. It is characterized by an increased heart rate, enhanced stroke volume, widened pulse pressure, and elevated cardiac output, all of which improve systemic oxygen delivery. However, the increased metabolic requirement may also predispose patients to relative tissue hypoxia, especially during periods of stress or exercise.
Pulmonary function is also affected by hyperthyroidism. Increased metabolic activity leads to hyperventilation and alterations in respiratory drive. Despite these changes, most hyperthyroid patients maintain normal or near-normal arterial oxygen tension under resting conditions. This suggests that compensatory cardiopulmonary mechanisms help preserve arterial oxygen saturation despite the increased oxygen demand.
Hemoglobin’s affinity for oxygen is influenced by several factors, including temperature, pH, and the concentration of 2,3-bisphosphoglycerate (2,3-BPG). In hyperthyroid states, elevated body temperature, increased 2,3-BPG levels, and a tendency toward respiratory alkalosis cause a rightward shift of the oxyhemoglobin dissociation curve (ODC), thereby facilitating oxygen release to tissues.
Oxygen saturation indicates the percentage of hemoglobin that is saturated with oxygen at the time of measurement. Values obtained through pulse oximetry (SpO?) form an important part of assessing a patient’s oxygenation status. In healthy individuals, normal oxygen saturation ranges from 97% to 99%, while a value of 95% is generally considered clinically acceptable in patients with normal hemoglobin levels. However, oxygen saturation alone does not reflect the patient's ability to ventilate adequately.