SLOT: Full Definition
What is TSH?
TSH — also called thyroid stimulating hormone or thyrotropin — is a hormone released by the pituitary gland in the brain. It is the body's primary signal telling the thyroid gland how much hormone to make. When circulating thyroid hormone is low, the pituitary releases more TSH to push the thyroid to produce more. When thyroid hormone is high, TSH falls to slow production down.
This feedback loop is why TSH is the single most ordered thyroid lab in conventional medicine. It is sensitive — small changes in thyroid hormone can move TSH well before patients feel different — but it is also indirect. It tells you what the pituitary thinks about thyroid status. It does not directly measure how much thyroid hormone is reaching your tissues.
At Modern Thyroid Clinic, TSH is one piece of a complete picture. We always pair it with Free T4, Free T3, Reverse T3, and antibodies — never relying on TSH alone.
Why does TSH matter?
TSH is what most clinicians use to:
- Screen for hypothyroidism and hyperthyroidism
- Adjust thyroid hormone medication doses
- Monitor stable thyroid disease over time
When TSH is high, the pituitary is shouting at an underactive thyroid (Hypothyroidism). When TSH is low, the thyroid is overproducing or being over-replaced with medication (Hyperthyroidism). When TSH looks "normal" but symptoms persist, the conversation has to widen.
Reference range vs. functional range
This is where many women fall through the cracks.
- Conventional lab reference range: roughly 0.4 to 4.5 mIU/L (varies by lab)
- Functional/optimal range used at MTC: 0.5 to 2.0 mIU/L
Most women feel best with TSH in the 0.5–2.0 range. By the time TSH climbs into the upper third of the conventional range — say 3.0 to 4.5 — many women are already symptomatic with fatigue, weight gain, hair loss, brain fog, and cold intolerance, but get told they are "normal." This mismatch between lab range and lived experience is one of the most common reasons women come to MTC.
What does an abnormal TSH mean?
- TSH high (above 4.5): primary hypothyroidism, often Hashimoto's. Confirm with Free T4, Free T3, and TPO antibodies.
- TSH high-normal (2.0–4.5) with symptoms: subclinical hypothyroidism or early Hashimoto's; warrants antibody testing and a fuller Full Thyroid Panel.
- TSH low (under 0.4): hyperthyroidism, Graves' disease, or thyroid medication over-replacement. Confirm with free hormones, Tsi, Trab.
- TSH normal but symptomatic: central/secondary thyroid dysfunction, conversion problems, or non-thyroid causes — never end the workup at TSH alone. This is the Functional Range Vs Reference Range conversation we have with nearly every new patient.
Common symptoms
Common questions
My TSH is 3.5 and my doctor says it's normal. Why do I still feel awful?
A TSH of 3.5 is technically inside the conventional reference range, but it sits well above the functional range of 0.5 to 2.0 where most women feel their best. Many patients with TSH in the 2.5 to 4.5 zone have early Hashimoto's, suboptimal T4-to-T3 conversion, or rising antibodies that have not yet pushed TSH out of range. At Modern Thyroid Clinic this is when we run the full panel — free T4, free T3, reverse T3, TPO and thyroglobulin antibodies — rather than calling it normal.
Can TSH be normal even with thyroid disease?
Yes — and this is one of the biggest reasons women feel dismissed. TSH can be in range while antibodies are quietly elevated for years before the gland fails. It can also be falsely normal in central hypothyroidism, in non-thyroid illness, with biotin supplements interfering with the assay, or after recent steroid use. A complete picture requires free T4, free T3, reverse T3, and antibodies — not TSH alone. We use the [full-thyroid-panel] as the standard intake at MTC.
What can falsely raise or lower TSH?
Several things shift TSH without reflecting true thyroid status. **Biotin supplements** can falsely lower TSH on many lab assays — stop biotin for at least 48 hours before testing. **Time of day** matters: TSH is highest in early morning and lowest in late afternoon. **Acute illness, pregnancy, recent steroid use, severe stress, and sleep deprivation** can all temporarily distort it. We standardize testing conditions and retest before making major changes when something looks off.
Think you might be dealing with this?
Talk to a Modern Thyroid Clinic specialist about your symptoms, labs, and next steps.
Book a Discovery CallThis content is for educational purposes only and is not medical advice. Consult a licensed clinician for diagnosis and treatment. Content on this page does not create a doctor-patient relationship with Modern Thyroid Clinic.