TSH

What TSH Measures

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.

What Affects This Biomarker

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.

In the Context of Peptide Protocols

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.

Deep Dive

The log-linear amplifier: why TSH is so sensitive

The pituitary does not respond to circulating thyroid hormone in a straight line. The relationship between free T4 and TSH is log-linear: as free T4 falls in equal steps, TSH rises geometrically rather than arithmetically. In practice, a change in free T4 too small to move the free T4 result out of its reference range can push TSH severalfold higher. The pituitary acts as a biological amplifier, and that amplification is why TSH is the single most sensitive first-line signal of primary thyroid dysfunction. It flags a drifting gland before free T4 visibly moves.

That same amplification is why TSH can mislead when read alone. Because TSH reports the pituitary's read on thyroid status rather than tissue-level thyroid hormone activity, it can look reassuringly normal in central (pituitary or hypothalamic) hypothyroidism, during non-thyroidal illness, and in the lag window after a dose change, when TSH can take 6 to 8 weeks to re-equilibrate. This is the mechanistic reason for the rule already noted on this page: read TSH with free T4 and free T3, not in isolation.

Where the reference range actually sits

The wide 0.4 to 4.5 band on most lab reports comes from population data that quietly included people with undiagnosed thyroid disease. When the NHANES III analysis (Hollowell et al., JCEM, 2002) excluded participants with thyroid antibodies, known thyroid disease, or axis-affecting medications, the disease-free reference population had a distribution centered near 1.4 mIU/L, with an upper bound well below the historical 4.5 ceiling. That analysis is the empirical origin of the sub-2 figure many optimization-minded clinicians track toward.

Age complicates this. Surks and Hollowell (JCEM, 2007) showed the 97.5th percentile of TSH rises meaningfully with age, from roughly 3.5 mIU/L in the 20s to around 7.5 mIU/L in the 80-plus group, so a TSH of 4 in an 80-year-old sits at a very different percentile than the same value at 30. And higher is not automatically worse late in life: Atzmon and colleagues (JCEM, 2009) reported that Ashkenazi Jewish centenarians and their offspring carried higher average TSH than younger controls. That is one reason chasing a sub-2 number in an older adult is not a universal goal and is a conversation to have with a clinician rather than a fixed rule.

Reading TSH between 2.5 and 10

The gray zone above the optimal target but below overt disease is where interpretation matters most. When TSH sits roughly between 4.5 and 10 mIU/L with a normal free T4, the pattern is described as subclinical hypothyroidism. AACE and ATA guidance (Garber et al., 2012) generally reserves treatment for TSH above 10, and individualizes decisions across the 4.5 to 10 range based on symptoms, positive TPO antibodies, age, and cardiovascular risk. TPO antibody status is the key modifier: antibody-positive individuals progress to overt hypothyroidism at a substantially higher annual rate, which is why an antibody panel adds context a lone TSH cannot. None of this is a basis for self-diagnosis. It is the framework a clinician uses, and it is the reason tracking the trend across serial draws is more informative than any single value.

Testing pitfalls that move the number

TSH is deceptively easy to mis-measure:

- **Time of day.** TSH follows a circadian rhythm, peaking overnight and reaching its nadir in the late afternoon to early evening. Morning draws can run meaningfully higher than afternoon draws in the same person, sometimes by 1 mIU/L or more. For a comparable trend, standardize the draw to the same early-morning window each time. - **Biotin.** High-dose biotin (B7), common in hair, skin, and nail supplements, interferes with many immunoassays and can falsely lower measured TSH while distorting free T4 and free T3, sometimes mimicking a hyperthyroid or Graves pattern. The FDA has issued formal safety communications on this. Hold biotin for 48 to 72 hours before testing. - **Acute illness.** During infection, injury, or hospitalization the axis is suppressed (non-thyroidal illness syndrome), so a TSH drawn while acutely sick may not reflect true baseline. Retest once recovered. - **Recent dose changes.** After any change in thyroid medication, TSH needs roughly 6 to 8 weeks to restabilize, so an early recheck reads the old steady state, not the new one.

Peptide and protocol tracking context

The growth hormone axis is the most relevant intersection for peptide users. GH and IGF-1 increase peripheral conversion of T4 to T3 by activating deiodinase enzymes, which can lower free T4 and nudge TSH downward after starting agents such as **tesamorelin**, **sermorelin**, **CJC-1295 with ipamorelin**, or **MK-677**. In people with borderline pituitary reserve, that shift can reveal a previously unrecognized central hypothyroidism pattern, which is exactly why a full thyroid panel, not TSH alone, is worth pulling at baseline and again 8 to 12 weeks into a GH-oriented protocol.

A few other tracking notes worth logging alongside the trend:

- **Estrogen** (oral HRT or high endogenous levels) raises thyroid-binding globulin and can increase the levothyroxine requirement in treated individuals, shifting TSH upward. - **Metformin** modestly lowers TSH in some hypothyroid and subclinical patients, an effect to keep in mind when reading a panel from someone running a metabolic stack. - **Selenium and iodine** status sit upstream of the whole axis. Both deficiency and excess of iodine can raise TSH, so a change in either supplement belongs in the log next to the lab value.

The through-line for protocol tracking is consistency: same lab, same assay, same early-morning fasted window, biotin held, and always in the context of free T4 and free T3. Discuss any persistent out-of-range trend with your clinician rather than adjusting a protocol on a single reading.

Peptides That Commonly Move TSH

Related Reading

• Thyroid Optimization Guide →

Conditions That Track TSH

Frequently Asked Questions

Where This Fits in Your Panel

TSH is one marker on a fuller panel. The Peptide Blood Work Checklist lays out the complete baseline panel, what to add by protocol type, and when to retest.

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