Abstract
Primary hypothyroidism is traditionally defined as intrinsic failure of the thyroid gland, most commonly attributed to autoimmune destruction or idiopathic atrophy. While this framework has guided diagnosis and treatment for decades, it may oversimplify the complex neuroendocrine, immune, and metabolic interactions that precede overt thyroid dysfunction. Emerging evidence suggests that immune dysregulation, chronic inflammation, hypothalamic–pituitary–adrenal (HPA) axis activation, nutrient insufficiency, mitochondrial dysfunction, and altered thyroid hormone signaling frequently predate biochemical thyroid failure. In this context, thyroid hypofunction may represent a downstream adaptive response rather than a primary isolated glandular disorder.
This narrative review reexamines the concept of “primary” hypothyroidism through an integrative systems-based lens, highlighting mechanisms of functional hypothyroidism, impaired peripheral hormone conversion, thyroid hormone resistance, and immune-mediated loss of tolerance. Clinical implications include recognition of patients who remain symptomatic despite normalization of thyroid-stimulating hormone levels and the limitations of levothyroxine monotherapy in select populations. Reframing hypothyroidism as a secondary manifestation of upstream physiologic stressors may improve diagnostic precision and support more personalized therapeutic strategies.
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