Triiodothyronine, commonly known as T3, is the active thyroid hormone that regulates the body’s energy use. This hormone is the primary driver of cellular activity across nearly every tissue and organ system. A well-functioning thyroid system is connected to metabolic health and a stable body weight. Low levels of T3 are associated with a decrease in the body’s energy expenditure, linking low T3 to metabolic changes that can lead to weight gain.
The Role of T3 in Metabolism
The thyroid gland produces two main hormones: thyroxine (T4) and triiodothyronine (T3). T4 is abundant but acts as a prohormone with little direct metabolic activity. T3 is the biologically potent form that interacts with cellular receptors to signal for increased energy production.
The conversion of T4 to T3 primarily occurs outside the thyroid gland, in peripheral tissues like the liver and kidneys. Enzymes called deiodinases remove an iodine atom from T4 to create active T3. This active T3 binds to nuclear receptors, influencing gene expression related to energy use, lipid metabolism, and carbohydrate processing. The collective effect of T3 activity is setting the Basal Metabolic Rate (BMR), which represents the calories the body burns to maintain life at rest.
How Low T3 Leads to Weight Changes
The physiological consequence of low T3 is a systemic reduction in the Basal Metabolic Rate. With less T3 available to stimulate cellular activity, the body burns fewer calories for maintenance functions like breathing, circulation, and temperature regulation. This decrease in energy expenditure creates an energy imbalance, meaning that normal caloric intake can exceed daily needs. The excess energy is then stored, often accumulating as adipose tissue.
Weight gain observed with low T3 involves more than just fat accumulation. A significant portion of the increase in body weight in individuals with hypothyroidism is due to the retention of salt and water. This fluid retention, known as myxedema, results from the accumulation of carbohydrates and proteins in the skin and connective tissues. The total body weight increase attributable to the hormone deficiency is often modest, ranging between five and ten pounds.
Low T3 levels also disrupt hormone systems that control appetite and energy storage. Reduced T3 can lead to altered signaling of leptin, the hormone that signals satiety, contributing to leptin resistance. This resistance may impair the body’s ability to regulate fat stores. Furthermore, low T3 can contribute to insulin resistance, where cells do not respond effectively to insulin, leading to more fat storage.
Clinical Contexts Where T3 Levels Fluctuates
Low T3 levels are most commonly observed in frank hypothyroidism, where the thyroid gland fails to produce sufficient hormones. The most frequent cause is Hashimoto’s thyroiditis, an autoimmune disease that attacks the thyroid gland. In this scenario, the pituitary gland releases a high level of Thyroid-Stimulating Hormone (TSH) to stimulate the failing thyroid, while T4 and T3 remain low.
T3 levels can also drop significantly in the absence of primary thyroid disease, a state referred to as Non-Thyroidal Illness Syndrome (NTIS) or euthyroid sick syndrome. This occurs during acute stress, severe illness, or starvation, where the body reduces the conversion of T4 to T3 to conserve energy. In NTIS, TSH and T4 levels may remain normal while T3 drops significantly. Another context is a problem with the conversion process itself, where the thyroid produces enough T4, but peripheral tissues struggle to convert it efficiently into active T3.
Diagnosis and Management Overview
Diagnosing low T3 involves blood tests to evaluate the entire thyroid axis. A healthcare provider typically measures TSH first, followed by free T4 and free T3 to determine the source of the imbalance. These results help distinguish between primary thyroid failure and conversion issues.
The standard medical approach for confirmed hypothyroidism is hormone replacement therapy, most often using synthetic levothyroxine (T4). The body converts this T4 into the necessary T3, aiming to restore a normal TSH level and metabolic state. If symptoms persist despite normalized TSH and T4, a physician may consider adding T3 supplementation, known as liothyronine. Anyone with concerns about low T3 or unexplained weight changes should seek guidance from a qualified healthcare professional for a precise diagnosis and personalized treatment plan.