Thyroid

TSH (Thyroid Stimulating Hormone)

Thyroid stimulating hormone is produced by the pituitary gland and signals the thyroid to produce T3 and T4 — the hormones that regulate metabolism, energy production, and body temperature. TSH operates on an inverse relationship: high TSH indicates the thyroid is underperforming (hypothyroidism), while low TSH suggests overactivity (hyperthyroidism).

Optimal Ranges

Clinical (NHS) Range

0.4-4.0 mIU/L

mIU/L

Performance-Optimised Range

1.0-2.0 mIU/L

mIU/L

The clinical range defines what is considered medically “normal” — broad enough to cover 95% of the population. The performance range reflects where research and clinical experience suggest most people feel and function at their best.

Why It Matters

Why TSH matters for performance

The thyroid is the body's metabolic thermostat. Even mildly elevated TSH — subclinical hypothyroidism — can cause fatigue, weight gain, cold intolerance, and depression that is frequently misattributed to ageing or overtraining. For men, thyroid dysfunction also impacts testosterone production and cardiovascular health. Standard clinical ranges for TSH are very broad (0.4-4.0 mIU/L), meaning you can be symptomatic at 3.5 and be told everything is 'normal'. Functional optimisation aims for a tighter range where most people feel their best.

Symptoms

Signs your levels may be off

Low / Deficiency

  • Low TSH (hyperthyroid symptoms):
  • Unexplained weight loss
  • Rapid heart rate and palpitations
  • Anxiety and tremors
  • Heat intolerance and sweating

High / Excess

  • High TSH (hypothyroid symptoms):
  • Fatigue and sluggishness
  • Unexplained weight gain
  • Cold intolerance
  • Dry skin and hair loss
  • Depression and brain fog
  • Constipation

Dietary Sources

Foods that support TSH levels

Iodine-rich foods (seaweed, cod, dairy)Selenium-rich foods (Brazil nuts — 2-3/day)Zinc-rich foods (oysters, beef, pumpkin seeds)Tyrosine sources (chicken, turkey, eggs)Iron-rich foods (supports T4 to T3 conversion)

Supplementation

Evidence-based supplementation approach

Selenium (200 mcg/day as selenomethionine) is the most evidence-based thyroid support nutrient — essential for T4 to T3 conversion and thyroid peroxidase function. Iodine supplementation (150-300 mcg/day) only if deficiency is confirmed — excess iodine can worsen thyroid conditions. Zinc (15-30 mg/day) supports thyroid hormone synthesis. Avoid soy isoflavones and excessive cruciferous vegetables if thyroid function is compromised. If TSH is persistently above 4.0, GP referral is recommended.

Research

Key study

Selenium supplementation in patients with autoimmune thyroiditis

Toulis KA, Anastasilakis AD, Tzellos TG, et al.

Thyroid (2010)

DOI: 10.1089/thy.2009.0351

Related Biomarkers

Hormones

Total Testosterone

Optimal: 20-30 nmol/L

Inflammation

Ferritin (Iron Stores)

Optimal: 80-150 ng/mL

Vitamins & Minerals

Vitamin D (25-OH)

Optimal: 100-150 nmol/L

Related Guides

Always Tired? The Blood Tests That Reveal Why

8 biomarkers behind fatigue

Complete Guide to Blood Tests for Men

What to test and why it matters

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This content is for educational purposes only and does not constitute medical advice. Your data suggests areas for optimisation, but any concerns should be discussed with a qualified healthcare professional. If your results flag values outside safe ranges, we recommend consulting your GP.