Hypothyroidism, a condition where the thyroid gland produces insufficient hormones, impacts various bodily functions. Bilirubin, a yellow pigment, is a natural byproduct of red blood cell breakdown. While these two aspects may seem unrelated, low thyroid hormone levels can lead to elevated bilirubin. This article explores how hypothyroidism influences bilirubin processing in the body.
Understanding Bilirubin Processing
Bilirubin’s journey begins with the natural breakdown of old red blood cells, primarily by macrophages in the spleen and other tissues. Hemoglobin converts to heme, then biliverdin, and finally unconjugated bilirubin. This unconjugated form is insoluble in water and must bind to albumin to travel through the bloodstream to the liver.
In the liver, unconjugated bilirubin undergoes a transformation. Liver cells contain the enzyme UDP-glucuronosyltransferase 1A1 (UGT1A1), which converts it into water-soluble conjugated bilirubin. This conversion is essential for effective excretion from the body. Once conjugated, it is secreted into bile, a digestive fluid, and then transported to the small intestine.
In the intestines, bacteria break down conjugated bilirubin into urobilinogen and stercobilin. Stercobilin is responsible for the brown color of stool. A portion of urobilinogen is reabsorbed into the bloodstream and excreted by the kidneys, giving urine its yellow hue. When this processing pathway is disrupted, bilirubin can accumulate in the blood. Yellowing of the skin and eyes becomes noticeable when bilirubin levels exceed approximately 3 mg/dL.
The Link Between Hypothyroidism and Bilirubin
Hypothyroidism can lead to elevated bilirubin levels, primarily by affecting the liver’s ability to process and excrete this pigment. Thyroid hormones regulate the metabolic activities of liver cells. When thyroid hormone levels are low, as in hypothyroidism, the overall metabolic rate in the liver can decrease, impacting its functions, including bilirubin metabolism.
One mechanism involves the impaired activity of the UGT1A1 enzyme within the liver. This enzyme converts unconjugated bilirubin into its water-soluble conjugated form. Reduced thyroid hormone can lead to decreased activity of this enzyme, meaning less unconjugated bilirubin is processed into its excretable form. This results in an elevation primarily of unconjugated bilirubin in the bloodstream.
Hypothyroidism can also contribute to cholestasis, a condition involving reduced bile flow from the liver. This reduction in bile secretion and bile salt excretion has been observed in hypothyroid states. Diminished bile flow can hinder the transport of conjugated bilirubin out of the liver and into the intestines. Hypothyroidism may also reduce gallbladder motility, leading to bile remaining in the organ for longer periods. These effects can contribute to the mild elevation of bilirubin seen in individuals with an underactive thyroid.
Recognizing Symptoms and Seeking Medical Advice
Elevated bilirubin levels often manifest as jaundice, a yellowing of the skin and the whites of the eyes. Changes in urine color, becoming darker, or stool color, appearing pale, can also indicate issues with bilirubin excretion. These physical signs warrant medical attention.
Individuals with hypothyroidism may experience a range of symptoms that can overlap with other conditions. Common indicators include persistent fatigue, unexplained weight gain, increased sensitivity to cold temperatures, constipation, and dry skin. Muscle aches, tenderness, and stiffness are also frequently reported. Seeking a medical evaluation is advisable if any of these signs are present.
Diagnosis of elevated bilirubin and hypothyroidism involves specific blood tests. To assess thyroid function, healthcare providers measure levels of thyroid-stimulating hormone (TSH) and thyroxine (T4). A high TSH level paired with a low T4 level usually confirms hypothyroidism. Bilirubin levels are assessed through a blood test that measures total bilirubin, as well as direct (conjugated) and indirect (unconjugated) bilirubin, to identify which form is elevated. Addressing the underlying hypothyroidism, commonly through daily oral levothyroxine medication, normalizes thyroid hormone levels and resolves the associated elevated bilirubin. This thyroid hormone replacement therapy is lifelong, with TSH levels periodically monitored to ensure proper dosage.