TSH Blood Test: Normal Ranges, Causes & What Your Results Mean

What Is TSH?

Thyroid stimulating hormone (TSH), also known as thyrotropin, is produced by the anterior pituitary gland and acts as the master regulator of thyroid function. TSH stimulates the thyroid gland to produce thyroxine (T4) and triiodothyronine (T3), which control metabolism, energy production, heart rate, body temperature, and brain development.

TSH operates via a negative feedback loop: when thyroid hormone levels are low, TSH rises to stimulate more production; when thyroid hormones are adequate, TSH falls. This makes TSH the most sensitive first-line test for thyroid dysfunction — it changes before T4 or T3 become abnormal.

Why Is TSH Tested?

• Thyroid disease screening — the single best initial test for both hypothyroidism and hyperthyroidism
• Monitoring thyroid medication — levothyroxine dose adjustments are guided by TSH
• Pregnancy thyroid screening — thyroid dysfunction affects foetal brain development
• Fatigue investigation — thyroid disease is one of the most common treatable causes of fatigue
• Weight changes — unexplained weight gain (hypothyroidism) or loss (hyperthyroidism)
• Mental health — thyroid dysfunction can mimic or worsen depression and anxiety

Normal Ranges

What Do High TSH Levels Mean? (Hypothyroidism)

• Hashimoto's thyroiditis — the most common cause of hypothyroidism in the UK
• Iodine deficiency — rare in the UK but common globally
• Post-thyroidectomy or radioiodine therapy
• Medications — lithium, amiodarone, interferon, immune checkpoint inhibitors
• Subclinical hypothyroidism — TSH 4.2–10 with normal FT4; affects 5–10% of the population
• Recovery from non-thyroidal illness — TSH may transiently rise during recovery from illness

What Do Low TSH Levels Mean? (Hyperthyroidism)

• Graves' disease — autoimmune stimulation of the thyroid; the most common cause of hyperthyroidism
• Toxic nodular goitre — autonomous thyroid nodule(s) producing excess hormone
• Thyroiditis — subacute, postpartum, or drug-induced; transient thyroid hormone release
• Excessive thyroid medication — over-replacement with levothyroxine
• First trimester pregnancy — hCG has mild TSH-like activity, transiently suppressing TSH
• Central hypothyroidism — rare; pituitary failure causes low TSH despite low thyroid hormones

How to Support Healthy Thyroid Function

• Adequate iodine — dairy products, seafood, and iodised salt are the main UK sources; 150 µg/day recommended
• Selenium — supports T4-to-T3 conversion and may reduce thyroid antibodies (2–3 Brazil nuts daily)
• Regular exercise — moderate activity supports metabolic rate and thyroid function
• Stress management — cortisol affects thyroid hormone conversion and may trigger autoimmune flares
• Take levothyroxine correctly — on an empty stomach, 30–60 minutes before food, with water only
• Avoid over-supplementing iodine — excess iodine can worsen Hashimoto's thyroiditis

When Should You Get Tested?

• Unexplained fatigue, weight changes, or mood disturbances
• Family history of thyroid disease
• Planning pregnancy or currently pregnant
• Taking thyroid medication (check every 6–12 months once stable)
• Taking medications known to affect thyroid (lithium, amiodarone)
• Other autoimmune conditions (type 1 diabetes, coeliac disease)

Which Lola Health Tests Include TSH?

TSH is included in Core Health, Vital Check, and Peak Insights. For a complete thyroid panel, add FT4, FT3, and thyroid antibodies (TPO, TgAb).