The pituitary signal to the thyroid — first-line thyroid test, but optimization requires free T4 + T3 context.
TSH (thyrotropin) is secreted by the pituitary to stimulate thyroid hormone production. Because the hypothalamic-pituitary-thyroid axis uses negative feedback, TSH rises when thyroid output is low and drops when it's high — making TSH a very sensitive marker of thyroid function.
Reference ranges on most lab reports are wide (0.4–4.5) and reflect population distribution, not optimal function. Functional-medicine and longevity clinicians typically target <2.0. Values in the 3–4.5 range are "normal" on paper but often coincide with subclinical hypothyroid symptoms.
TSH alone is insufficient for full thyroid assessment. Always pull free T4, free T3, and reverse T3 alongside it for the full picture.
TSH is influenced by: thyroid gland function, pituitary health, iodine status, selenium and zinc, chronic inflammation, acute illness (non-thyroidal illness syndrome), recent contrast-dye exposure, and pharmacologic agents — levothyroxine suppresses TSH; lithium, amiodarone, and estrogen can raise it.
Track TSH annually for general health and quarterly if any thyroid dysfunction is suspected. On GH protocols (tesamorelin, HGH), thyroid status can shift — GH increases peripheral T4→T3 conversion, which can lower TSH slightly and warrant a full panel. Sub-2.0 TSH is fine if free T4 and free T3 are both in range.
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Start tracking →Informational only — not medical advice. Reference ranges vary by lab and individual context. Work with a licensed provider to interpret your specific results.