Allison Sayre, MSN, WHNP and Corey Schuler, PhD, FNP, CNS
Levothyroxine has long been the standard treatment for hypothyroidism. The rationale is simple. Replace missing thyroid hormone, normalize thyroid stimulating hormone, and rely on the body to convert thyroxine (T4) into the active hormone triiodothyronine (T3) as needed. For many people, this approach works well. Symptoms improve, energy returns, and laboratory values settle into reference ranges.
Thyroid hormone signaling in hypothyroidism can be nuanced. A meaningful subset of individuals (often estimated at 10 to 20 percent), continue to experience fatigue, cognitive dullness, mood changes, weight challenges, or a sense that their metabolic momentum never fully returns, even though their labs are described as normal. [1] This disconnect raises an important question, and that is, are we actually restoring thyroid hormone signaling in hypothyroidism, or are we merely restoring thyroid hormone numbers?
What Thyroid Hormone Signaling in Hypothyroidism Really Means
Thyroid hormones do more than circulate in the bloodstream. They act as signals that enter cells and regulate gene expression, metabolic rate, heat production, lipid handling, cognition, and mood. T4 is the most abundant hormone measured in blood tests, but T3 is the biologically active form that actually drives these effects at the cellular level. [1]
In a healthy system, the thyroid gland itself contributes directly to T3 production, while most circulating T3 is generated through conversion of T4 to T3 in peripheral tissues by deiodinase enzymes. [1] This design allows different tissues to fine-tune how much active hormone they receive.
When hypothyroidism develops, most commonly due to autoimmune thyroiditis or thyroid removal, this balance changes. Treatment with levothyroxine restores circulating T4 and normalizes thyroid-stimulating hormone (TSH), but it also eliminates the thyroid gland’s direct contribution to T3. [1] That shift has important implications for how signaling is restored across the body.
The Limits of a TSH-Centered Model
TSH has long been treated as the primary marker of treatment success. When TSH is normal, the assumption is that tissues are receiving adequate thyroid hormone. This assumption depends on the idea that peripheral conversion of T4 to T3 will fully compensate for the loss of thyroidal T3 secretion.
Evidence suggests that this compensation may be incomplete in some individuals. [1] On average, people treated with levothyroxine tend to have higher circulating T4 levels and lower circulating T3 levels than individuals with intact thyroid function, even when TSH is within range. In a notable minority, T3 levels fall near or below the lower end of the reference range. [2]
The pituitary gland is especially efficient at converting T4 to T3. This efficiency helps explain why TSH can normalize even when T3 availability in other tissues is relatively lower. [1] In this scenario, laboratory feedback may signal adequacy while peripheral tissues experience less robust thyroid hormone signaling.
Persistent Symptoms Have a Physiological Basis
Individuals who remain symptomatic on levothyroxine often report a consistent cluster of experiences. These include mental fatigue, difficulty concentrating, memory lapses, sleep disturbances, anxiety, low mood, and reduced quality of life. These symptoms are sometimes grouped under the term brain fog. [1][3]
Carefully designed studies comparing levothyroxine-treated individuals with matched control populations repeatedly show differences in psychological well-being and cognitive performance, even when TSH levels are normal. These differences are not fully explained by age, depression, or other medical conditions. [1][2]
Metabolic differences have also been observed. On average, treated individuals tend to weigh more and may have a lower basal metabolic rate than expected. Lipid levels can remain subtly abnormal despite appropriate therapy and concurrent lipid-lowering treatment. [1][2] Taken together, these findings suggest that residual symptoms are rooted in biology rather than effort, motivation, or perception.
Why T3 Availability Matters
From a signaling perspective, T3 is the hormone cells actively use. It is the form that binds nuclear thyroid hormone receptors and directly regulates gene transcription. As a result, even relatively small differences in T3 availability can translate into meaningful differences in gene expression, mitochondrial activity, and metabolic output across tissues. These effects are graded rather than binary, meaning subtle shifts can still have physiologic consequences. [1][4]
The ability to convert T4 into T3 varies considerably between individuals. This process depends on deiodinase enzyme activity, tissue-specific regulation, and genetic variation. Certain deiodinase polymorphisms reduce conversion efficiency and may leave some individuals less able to rely solely on T4 therapy to meet tissue-level T3 needs. [1][4]
Importantly, impaired T4-to-T3 conversion is not driven by genetics alone. Deiodinase activity is sensitive to broader physiologic context. Nutrient inadequacies (particularly of iodine, selenium, iron, and zinc) can impair enzymatic conversion. [5] Both physiological and psychological stress have been shown to impact thyroid hormone secretion. [6][7] Caloric restriction, low carbohydrate availability, acute illness, and systemic inflammation can downregulate peripheral conversion as part of an energy-conservation response. [7] In these settings, normal circulating T4 and TSH levels may coexist with reduced tissue-level T3 signaling.
Residual thyroid tissue also appears to matter. Individuals with partial thyroid function, whether due to incomplete autoimmune destruction or partial thyroidectomy, tend to maintain higher circulating T3 levels than those with little or no remaining thyroid tissue, even when receiving similar levothyroxine doses. [1] This observation reinforces the idea that the thyroid gland itself plays a unique role in supporting overall T3 balance, beyond what peripheral conversion alone can fully replace.
Is Simply Increasing T4 the Solution?
One historical approach to persistent symptoms has been to increase the levothyroxine dose until TSH becomes suppressed. While this strategy can raise T3 levels and improve quality of life in some cases, it carries important risks.
Chronically suppressed TSH has been associated with higher rates of cardiovascular events, arrhythmias, bone loss, and fractures. [1][8] Improving symptoms by pushing the system into biochemical excess often comes at the expense of long-term safety. For most individuals, the goal is not maximal hormone exposure. It is balanced signaling.
A Broader Integrative Perspective
Persistent symptoms should not automatically be attributed to thyroid hormone signaling alone. Iron deficiency, vitamin B12 or folate insufficiency, vitamin D deficiency, sleep disorders, metabolic dysfunction, hormonal imbalance, and immune activity related to autoimmune thyroid disease can all contribute to fatigue and cognitive symptoms. [1]
Beyond contributing to symptoms directly, many of these factors can also impair peripheral conversion of T4 to T3. Inadequate nutrient status, persistent physiologic stress, low caloric or carbohydrate intake, systemic inflammation, and illness can downregulate deiodinase activity and shift thyroid hormone metabolism toward conservation rather than activation. In these contexts, normal TSH and T4 levels may coexist with reduced tissue-level T3 signaling. [4-7]
An integrative approach acknowledges this complexity. It begins with validating symptoms, ruling out potential causes, identifying contributors that may blunt T4-to-T3 conversion or increase metabolic demand, and then asking whether thyroid hormone signaling, rather than TSH normalization alone, has truly been restored.
The Bigger Picture
Taken together, emerging evidence suggests that hypothyroidism is not simply a condition of low hormone levels, but one of altered signaling across interconnected systems. For many individuals, levothyroxine successfully restores that signaling. For others, factors such as impaired T4-to-T3 conversion, metabolic stress, nutrient inadequacy , immune activity, or altered energy allocation may limit full physiologic recovery, even when laboratory targets are met. [1]
Recognizing this complexity does not undermine standard therapy. It refines it. A more integrative, individualized approach creates space for shared decision-making, thoughtful evaluation of contributing factors, and, when appropriate, carefully monitored adjustments in therapy. Effective thyroid care ultimately aims for more than normal lab values. It seeks to restore functional capacity, cognitive clarity, and metabolic resilience in a way that reflects the dynamic biology of the human system. [1]
Disclaimer:
The information provided is for educational purposes only. Consult your physician or healthcare practitioner if you have specific questions before instituting any changes in your daily lifestyle including changes in diet, exercise, and supplement use.
Allison Sayre, MSN, WHNP is a board-certified women’s health nurse practitioner with advanced expertise in hormonal health, integrative gynecology, and patient-centered care across the lifespan. She holds a Master of Science in Nursing and has served as both a clinical provider and educator in functional and conventional women’s health settings. At ARG, Allison contributes to medical education, clinical protocol development, and strategic content that supports the evolving needs of women's healthcare practitioners.
Corey Schuler, PhD, FNP, CNS has dedicated his career to advancing the science and clinical art of integrative medicine and serves as director of medical affairs for Allergy Research Group. He is a family nurse practitioner and practices holistic primary care at Synergy Family Physicians in White Bear Lake, Minnesota.
