TSH
Thyroid-stimulating hormone, the first-line thyroid screen. Always paired with a free T4 when abnormal.
TL;DR. Normal adult: ~0.4-4.0 mIU/L. Tested early in every low-T workup because thyroid dysfunction mimics hypogonadism. High TSH = hypothyroid (mostly Hashimoto's); low TSH = hyperthyroid. Always interpret with free T4 for a complete picture.
Definition
TSH (thyroid-stimulating hormone, thyrotropin) is a glycoprotein hormone secreted by the anterior pituitary under hypothalamic TRH control. It binds thyroid-follicular-cell receptors and drives thyroid-hormone synthesis and release. Because the pituitary output responds log-scale to small changes in circulating free T4, TSH is the most sensitive first-line screen in ambulatory adults. In hospitalized or critically-ill patients, TSH is less reliable, use free T4 directly.
Reference range (adults)
| Range (mIU/L) | Interpretation |
|---|---|
| 0.4-4.0 | Normal (most labs) |
| 2.5-4.0 | High-normal; some advocate treating if symptomatic with TPO+ |
| 4-10 with normal free T4 | Subclinical hypothyroidism |
| >10 | Overt primary hypothyroidism (usually treat) |
| 0.1-0.4 | Subclinical hyperthyroidism |
| <0.1 | Overt hyperthyroidism (usually) |
Ranges are laboratory- and age-dependent. TSH rises normally with age; NHANES age-specific 97.5th-percentile data (Surks 2007) put the upper limit closer to ~5-6 mIU/L in the 70s and up to ~7-7.5 in adults 80+.
What high TSH means
- Hashimoto's thyroiditis (most common cause of hypothyroidism in iodine-sufficient areas).
- Iodine deficiency, rare in iodized-salt regions.
- Post-radioiodine ablation or thyroidectomy without adequate levothyroxine.
- Drugs: lithium, amiodarone, interferon-α, tyrosine-kinase inhibitors.
- Levothyroxine underdosing or malabsorption (celiac disease, proton-pump inhibitors reduce absorption).
- Symptoms: fatigue, weight gain, cold intolerance, dry skin, hair thinning, constipation, low mood, low libido.
What low TSH means
- Graves disease (autoimmune hyperthyroidism).
- Toxic multinodular goiter or toxic adenoma.
- Thyroiditis (subacute, postpartum), transient hyperthyroid phase.
- Exogenous levothyroxine overdosing.
- Iodine load (amiodarone, contrast dye).
- Central hypothyroidism (pituitary disease), TSH low or inappropriately normal with low free T4.
- Severe illness (non-thyroidal illness syndrome), transient.
- Symptoms: weight loss, heat intolerance, tachycardia, tremor, anxiety, insomnia, frequent stools.
When to test
- Baseline in any workup of low testosterone, fatigue, weight change, or mood change.
- Before starting TRT (thyroid dysfunction mimics low T).
- 6-8 weeks after any dose change of levothyroxine.
- Routinely every 2-3 years in asymptomatic adults; annually in women >50.
Related tests
FAQ
- What is the normal TSH range?
- Approximately 0.4-4.0 mIU/L for most adult reference labs, though some advocate an upper limit closer to 2.5 based on iodine-sufficient healthy cohorts. Values 4-10 with normal free T4 are subclinical hypothyroidism; below 0.1 with normal free T4 is subclinical hyperthyroidism. Interpret always with free T4.
- Why is TSH tested in a low-testosterone workup?
- Because hypothyroidism produces fatigue, low libido, low mood, cold intolerance, and weight gain, a symptom cluster easily confused with low T. Treating hypothyroidism often resolves the low-T-like symptoms completely. Hyperthyroidism also raises SHBG, which can mask low free T on total-T-only panels. TSH is a one-test screen that prevents misclassification.
- What does high TSH mean?
- High TSH with low free T4 = overt primary hypothyroidism. High TSH with normal free T4 = subclinical hypothyroidism (treat if TSH >10, symptomatic, or TPO-antibody-positive). Common causes: Hashimoto's thyroiditis, iodine deficiency, post-radioablation, post-surgery, lithium, amiodarone, and thyroid-hormone replacement underdosing.
- What does low TSH mean?
- Low TSH with elevated free T4/T3 = hyperthyroidism (Graves, toxic nodule, thyroiditis). Low TSH with low free T4 = central hypothyroidism or recent thyroid-hormone change. Low TSH with normal free T4 = subclinical hyperthyroidism. Common causes include exogenous levothyroxine, iodine load, amiodarone, severe illness (low-T3 syndrome), and pituitary disease.
- How often should I recheck TSH?
- Baseline in any hormone workup. In stable asymptomatic adults, every 2-3 years. After dose changes of levothyroxine, recheck at 6-8 weeks. With an abnormal result, confirm with a second draw 2-4 weeks later before starting treatment.