Why this panel matters for peptide users
Two reasons the thyroid panel deserves your attention specifically as a peptide user:
- GH-axis peptides shift conversion. CJC-1295, ipamorelin, sermorelin, tesamorelin, MK-677 — all of these raise IGF-1 and (variably) shift the T4-to-T3 conversion ratio. The published evidence is mixed but the signal is real enough that anyone running GH-axis peptides for > 3 months should baseline and recheck thyroid.
- GLP-1s and weight loss change thyroid demand. Significant fat loss reduces the metabolic load the thyroid is supplying. TSH may rise modestly during active weight loss as the system rebalances; this isn’t hypothyroidism, it’s adaptation. Wait until weight has been stable for 8+ weeks before drawing conclusions from a single TSH bump.
The full panel
TSH (thyroid-stimulating hormone)
Pituitary signal telling the thyroid to make hormone. Inversely correlated with thyroid output — high TSH means the pituitary is shouting because thyroid output is low; low TSH means the thyroid is producing enough (or too much) and the loop has closed. The functional-medicine optimal range (0.5-2.0) is tighter than the lab range (0.4-4.0); a TSH of 3.5 isn't 'normal' if you also have low free T3 and symptoms.
Free T4
The 'storage form' of thyroid hormone. Most circulating thyroid hormone is T4, which gets converted peripherally to T3 (active form) in liver, kidney, and other tissues. Free T4 alone tells you the supply side is working. It does not tell you how much active hormone tissues are seeing — that's free T3.
Free T3
The active hormone — what tissues actually use. Most labs run this only on request; it's not in the default thyroid panel. ASK FOR IT. Low free T3 with normal TSH and free T4 is the conversion-impaired pattern (next section). High free T3 with suppressed TSH is hyperthyroidism. Symptoms track free T3 closely; TSH lags.
Reverse T3 (rT3)
The 'inactive' isomer of T3 — your body's brake on thyroid signaling. Stress, illness, calorie restriction, and inflammation all push T4 conversion toward rT3 instead of T3. Elevated rT3 with normal TSH and free T4 is the 'low T3 syndrome' or 'euthyroid sick syndrome' pattern — the lab report won't flag it because every individual number is in range. Calculate the free-T3-to-rT3 ratio: above 0.20 is the goal; below suggests conversion impairment.
TPOAb (thyroid peroxidase antibodies)
The autoimmune marker. Elevated TPOAb is the signature of Hashimoto's thyroiditis, which is by far the most common cause of hypothyroidism in developed countries. Order it once — antibody status doesn't change quickly. Positive TPOAb means autoimmune activity even if TSH is currently normal; the thyroid is being slowly attacked and thyroid output will likely decline over time.
TgAb (thyroglobulin antibodies)
Second autoimmune marker. Less sensitive than TPOAb but catches a subset of cases TPOAb misses. Order alongside TPOAb on the baseline draw; positive TgAb without positive TPOAb is uncommon but real.
The conversion-impaired pattern
This is the pattern lab reports won’t flag and most primary-care clinicians will miss. Worth understanding in detail.
Pattern: low T3 syndrome
- TSH: normal (often low-normal, 0.5-1.5)
- Free T4: normal (often mid-range)
- Free T3: low or low-normal
- Reverse T3: high or high-normal
- Free T3 / rT3 ratio: < 0.15
- Symptoms: fatigue, cold intolerance, constipation, low mood, slow recovery — the classic hypothyroid phenotype, but TSH says everything is fine
Common causes: chronic calorie restriction (very common in GLP-1 users), chronic over-training, sleep debt, chronic inflammation, low ferritin, low selenium. Also liver dysfunction, since most T4-to-T3 conversion happens there. Treatable by addressing the upstream cause; sometimes requires T3-containing thyroid supplementation under clinical supervision.
Reading the panel
Don’t treat TSH alone. A TSH-only panel can miss low-T3 syndrome, autoimmune Hashimoto’s in the early phase, and conversion problems entirely. Free T4 + free T3 + (at baseline) TPOAb + reverse T3 is the minimum useful workup if anything is symptomatic.
Optimal ranges differ from lab ranges. Lab ranges are population-based — the 2.5th to 97.5th percentile of whoever the lab sampled. Functional-medicine optimal ranges are tighter and reflect where most people feel best. A TSH of 3.5 is “in range” on paper but symptomatic for many people. Same for free T3 in the bottom third of the range.
Symptoms outrank numbers. If you feel terrible and your TSH is 4.2, the answer isn’t “you’re fine because your TSH is 4.2.” The answer is to look at the rest of the panel and see if there’s a coherent picture. If it’s incoherent (symptoms but every number is normal), it might not be thyroid at all — adrenal, iron, sleep, or something else entirely.
When to draw
- Morning, fasted. TSH peaks overnight and falls during the day; afternoon draws can read 30-50% lower. Standardize to 7-10am for comparable reads across time.
- Hold biotin for 48-72 hours before draw. High-dose biotin (B7) supplementation interferes with the immunoassay and can produce falsely-low TSH and falsely-high free T4/T3. If you take a B-complex with biotin or a hair/nail formula, hold for at least 48 hours.
- Wait 6-8 weeks after starting/changing thyroid medication for steady state. Earlier draws don’t reflect equilibrium.
- Wait 8+ weeks of stable weight before reading TSH on or after a major weight-loss phase. The TSH bump during active loss isn’t pathology.
How often to retest
- Baseline before starting GH-axis peptides: full panel including reverse T3 + antibodies. Repeat at 3 months and 6 months.
- On stable thyroid medication: annually, unless symptoms change.
- Asymptomatic adult, no peptide context: every 2-3 years as part of routine bloodwork; sooner if symptoms emerge.
What Juno can do with these values
Thyroid markers aren’t in the seeded biomarker library yet — TSH, free T4, free T3, reverse T3, and the two antibody markers will be added in a future seed update. Once they’re in, /account/labs will support pasting and analyzing them with the same workflow as the markers already covered (testosterone, IGF-1, HbA1c, hs-CRP, ALT, vitamin D).
For now, the framework above is editorial-only. If you want to track the values, paste them into a notebook or spreadsheet alongside your stack changes; the patterns (especially conversion-impaired ratios) become obvious over multiple draws even without app support.