A manufactured form of the active thyroid hormone. It restores thyroid status reliably and is the only way to raise the active hormone directly. For symptoms persisting on the storage hormone alone, average gains are absent though the most symptomatic have tended to improve. Harms follow from too much hormone; serious ones cluster around unlicensed or badly made product. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Thyroid-stimulating hormone (TSH) | 0.5–2.0 mIU/L | The primary dosing target; suppression drives bone and rhythm risk |
| Free thyroxine (free T4) | 0.8–1.2 ng/dL | Confirms the storage hormone has not been cut too far when liothyronine is added |
| Free triiodothyronine (free T3) | 3.0–4.0 pg/mL at trough | The hormone actually being given; the only direct check on delivered dose |
| Sex hormone-binding globulin (SHBG) | 30–90 nmol/L (women), 20–60 nmol/L (men) | A liver-derived readout of tissue thyroid hormone action, rising with overtreatment |
| LDL cholesterol | Below 100 mg/dL, or unchanged from personal baseline | Tracks the hepatic response to thyroid hormone and falls with adequate dosing |
| Resting heart rate | 55–70 bpm | The most accessible early sign of excess dosing |
| Lumbar spine and femoral neck bone mineral density | T-score above −1.0 | Detects the bone loss that follows sustained suppression |
| Ferritin | 50–100 ng/mL | Iron deficiency reproduces the fatigue attributed to inadequate conversion |
| 25-hydroxyvitamin D | 40–60 ng/mL | Another common cause of fatigue and low mood that would otherwise be misattributed |
| Corrected QT interval on electrocardiogram | Below 450 ms (men), below 460 ms (women) | Screens for the conduction abnormality that makes rhythm disturbance likely |
Cadence: Thyroid function at 6 weeks, at 12 weeks, then every 6 months once stable, always drawn before the morning dose. Lipids and sex hormone-binding globulin at 6 months; bone densitometry every 24 months if the stimulating hormone runs below 0.4 mIU/L; heart rate at every review.