# TSH

Source: https://synapse-lifestyle.com/en/reference-guide/biomarkers/tsh

Unit: mU/L

## What is TSH?

TSH is the hormone your brain uses to drive your thyroid. A high value often means the thyroid needs a nudge; a low one, the opposite. Your thyroid regulates your energy, weight and mood. TSH is the first, sensitive signal of whether that control is in balance.

## Clinical frame

TSH is the **most sensitive first-line test** of thyroid function: because of the log-linear feedback, a small fall in free T4 causes a much larger rise in TSH, so TSH deviates before free T4 does. Free T4 is only measured once TSH falls outside the reference interval [1].

An elevated TSH with a normal free T4 is called **subclinical hypothyroidism** and occurs in up to 10% of adults. The risk of heart failure and coronary death rises mainly from a TSH of 10 mIU/L. Mildly elevated values mainly call for follow-up, and in older adults the upper limit is physiologically higher [2].

A suppressed TSH with a normal free T4 points to **subclinical hyperthyroidism**, with more atrial fibrillation, bone loss and fractures, most clearly at a TSH below 0.1 mIU/L and above the age of 65 [3]. Because reference intervals differ between assays and TSH can fluctuate temporarily, a repeat measurement confirms the finding [1].

## What moves this value?

The thyroid picture is nuanced and moves with heredity, iodine, stress and sometimes autoimmune processes.

This is mainly a marker to follow, not to adjust yourself. Take an unusual value to your doctor as a talking point.

## What this connects

- [FT4](https://synapse-lifestyle.com/en/reference-guide/biomarkers/ft4)

Developed and reviewed with the Synapse [medical advisory board](https://synapse-lifestyle.com/en/advisory-board).

## References

1. Van Uytfanghe, K. et al. Thyroid stimulating hormone and thyroid hormones (triiodothyronine and thyroxine): an American Thyroid Association-commissioned review of current clinical and laboratory status. Thyroid 33, 1013–1028 (2023). https://doi.org/10.1089/thy.2023.0169
2. Biondi, B., Cappola, A. R. & Cooper, D. S. Subclinical hypothyroidism: a review. JAMA 322, 153–160 (2019). https://doi.org/10.1001/jama.2019.9052
3. Biondi, B. & Cooper, D. S. Subclinical hyperthyroidism. N Engl J Med 378, 2411–2419 (2018). https://doi.org/10.1056/NEJMcp1709318

- [Read more in the reference guide](https://synapse-lifestyle.com/en/reference-guide/hormones#Consequence-CSThyroid-FTSH)
- [All biomarkers in the guide](https://synapse-lifestyle.com/en/reference-guide#biomarkers)

The colour zones are personal: they depend on age, sex, risk and history, and appear in the patient's own report.

Insights to discuss with the patient, not a diagnostic instrument.