What it measures.
TSH (Thyroid Stimulating Hormone) is produced by the pituitary gland to regulate thyroid hormone production. It's the most commonly ordered thyroid test, but interpretation requires nuance—a 'normal' TSH can mask suboptimal thyroid function.
TSH measures the blood concentration of thyroid-stimulating hormone, secreted by the anterior pituitary gland. TSH acts on the thyroid gland to stimulate production and release of T4 and T3 thyroid hormones.
Why it matters.
Thyroid hormones control metabolism, energy production, body temperature, heart rate, and cognitive function. TSH abnormalities affect 5-20% of adults, with subclinical dysfunction often going undetected. Optimal thyroid function is essential for metabolic health and longevity.
Physiology.
The hypothalamic-pituitary-thyroid (HPT) axis operates through negative feedback. When thyroid hormones drop, the hypothalamus releases TRH, which triggers pituitary TSH release. TSH then stimulates the thyroid to produce T4 and T3. When hormone levels are adequate, TSH decreases. This feedback loop means TSH rises when thyroid output is insufficient (hypothyroidism) and falls when it's excessive (hyperthyroidism).
Testing & preparation.
How to prepare
- Test in the morning—TSH has circadian variation (highest in early morning)
- Fasting not strictly required but provides consistency
- Note any biotin supplements (can interfere with some assays—stop 2-3 days before)
- Continue thyroid medication unless instructed otherwise
When to test
Annual screening for adults, especially women over 35. More frequently if on thyroid medication or with symptoms.
How often
Annually for screening; every 6-8 weeks when adjusting thyroid medication; every 3-6 months once stable.
Interpretation.
High tsh
Common causes:
- Primary hypothyroidism (thyroid gland dysfunction)
- Hashimoto's thyroiditis (autoimmune)
- Iodine deficiency
- Medications (lithium, amiodarone)
- Pituitary tumors (rare—TSH-secreting)
- Recovery from non-thyroidal illness
- Aging (TSH reference ranges shift higher)
Implications:
- Thyroid gland not producing enough hormone
- Fatigue, weight gain, cold intolerance, constipation
- Cognitive slowing, depression
- Elevated cholesterol
- May progress to overt hypothyroidism if untreated
Low tsh
Common causes:
- Hyperthyroidism (Graves' disease, toxic nodules)
- Excessive thyroid hormone replacement
- Central hypothyroidism (pituitary/hypothalamic dysfunction)
- Non-thyroidal illness (sick euthyroid syndrome)
- Medications (corticosteroids, dopamine)
- Early pregnancy
Implications:
- May indicate overactive thyroid
- Anxiety, tremor, weight loss, heat intolerance
- Increased heart rate and risk of atrial fibrillation
- Bone loss risk with sustained suppression
- If on thyroid medication: dose may need reduction
Optimization.
Diet
- Ensure adequate iodine intake (seafood, dairy, iodized salt)
- Include selenium-rich foods (Brazil nuts, fish)—supports T4 to T3 conversion
- Zinc supports thyroid function (oysters, beef, pumpkin seeds)
- Avoid excessive raw cruciferous vegetables if iodine-deficient
- Avoid extreme caloric restriction (impairs thyroid function)
Lifestyle
- Manage chronic stress (cortisol impairs T4 to T3 conversion)
- Prioritize quality sleep (7-8 hours)
- Avoid overtraining (excessive exercise can suppress thyroid)
- Reduce exposure to endocrine disruptors
- Address iron deficiency (required for thyroid hormone synthesis)
Supplements
- Selenium: 200mcg daily if deficient (supports conversion and may lower antibodies)
- Iodine: only if deficient—excessive iodine can worsen autoimmune thyroid
- Vitamin D: optimize to 40-60 ng/mL (deficiency linked to autoimmune thyroid)
- Zinc: 15-30mg if deficient
FAQs.
Is my TSH of 3.5 mIU/L really 'normal'?
While 3.5 falls within most lab reference ranges, research suggests optimal thyroid function occurs at lower TSH levels (0.5-2.5 mIU/L). Many people with TSH in the upper 'normal' range have symptoms of suboptimal thyroid function. The key is correlating your TSH with symptoms and other thyroid markers.
Why doesn't my doctor test Free T3?
Many practitioners rely on TSH alone, but this can miss important patterns. Some people have normal TSH and T4 but low T3 due to poor conversion. If you have thyroid symptoms despite 'normal' TSH, request a complete panel: TSH, Free T4, Free T3, and thyroid antibodies.
Can stress affect my TSH?
Yes, chronic stress elevates cortisol which can impair T4 to T3 conversion, leading to symptoms of hypothyroidism even with normal TSH. Stress can also trigger or worsen autoimmune thyroid disease. Address stress as part of thyroid optimization.
Should TSH be tested fasting?
Fasting isn't strictly required, but TSH has circadian variation—it's highest in early morning and can vary by 50% throughout the day. For consistent monitoring, test at the same time of day, ideally in the morning.
Why did my TSH change after weight loss or dietary changes?
Thyroid function is sensitive to energy availability. Severe caloric restriction can lower T3 and raise TSH as the body conserves energy. Conversely, addressing nutrient deficiencies (iodine, selenium, zinc) can improve thyroid function. Weight loss can also improve thyroid autoimmunity.