TSH (Thyroid Stimulating Hormone)

Hormone that regulates thyroid function

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What is TSH (Thyroid Stimulating Hormone)?

TSH (Thyroid Stimulating Hormone) is a hormone produced by your pituitary gland that regulates your thyroid gland's production of thyroid hormones (T4 and T3). Think of TSH as the "thermostat"for your thyroid:when thyroid hormone levels drop, your pituitary releases more TSH to tell the thyroid to produce more hormone. When thyroid hormone levels are sufficient, TSH drops.

Here's the counterintuitive part that confuses many people:high TSH means your thyroid is underactive (hypothyroidism), while low TSH means your thyroid is overactive (hyperthyroidism). It's an inverse relationship—TSH goes up when thyroid function goes down, like a thermostat cranking up the heat when the house is too cold.

TSH is the single most important initial screening test for thyroid function. However, TSH alone doesn't tell the full story—you also need Free T4 and Free T3 to understand what's actually happening at the tissue level. Some people have normal TSH but low Free T3 due to conversion problems, and they'll have hypothyroid symptoms despite "normal"labs.

Why TSH Alone Isn't Enough

Central hypothyroidism:Pituitary gland failure causes low TSH + low thyroid hormones (rare but serious)
T4→T3 conversion problems:Normal TSH/T4 but low Free T3 due to stress, selenium deficiency, chronic illness
Subclinical hypothyroidism:Elevated TSH with normal Free T4—controversial whether to treat
Hashimoto's (autoimmune) thyroiditis:TSH may be normal early on but TPO antibodies are elevated

Optimal vs Standard Ranges

Range Type	Level	Significance
Optimal (Longevity)	0.5-2.5 mIU/L	Best metabolic function, energy, and symptom control. Many functional medicine doctors target this range.
Low-Normal (Borderline Low)	2.5-4.0 mIU/L	Within standard range but may have subtle hypothyroid symptoms. Consider retesting in 3-6 months or checking antibodies.
Subclinical Hypothyroidism	4.0-10.0 mIU/L	Thyroid struggling but Free T4 still normal. Controversial treatment threshold. Consider treatment if symptomatic or trying to conceive.
Overt Hypothyroidism	>10.0 mIU/L	Clear thyroid failure;requires treatment. Free T4 will be low. Start levothyroxine and retest in 6-8 weeks.

Standard lab range: energy

How to Optimize TSH (Thyroid Stimulating Hormone)

1. Hyperthyroidism (Overactive)

<0.4 mIU/L

2. Thyroid producing too much hormone. Check Free T4/T3 (will be high). Evaluate for Graves'disease or thyroiditis.

Treat Underlying Hypothyroidism (if TSH >2.5-4.0)

3. Levothyroxine (Synthroid, generic T4):Standard treatment. Start 25-50 mcg daily, titrate based on TSH every 6-8 weeks until TSH is 0.5-2.5 mIU/L. Take on empty stomach, 30-60 minutes before food. | T4/T3 combination therapy:Some patients feel better adding T3 (liothyronine/Cytomel) to levothyroxine, especially if Free T3 remains low. Typical ratio:4:1 or 5:1 (T4:T3). | Natural desiccated thyroid (NDT):Armour Thyroid, NP Thyroid. Contains both T4 and T3 from pig thyroid. Some patients prefer it but evidence is mixed.

Optimize Thyroid-Supporting Nutrients

4. Iodine (150-300 mcg/day):Essential building block of thyroid hormones (T4 has 4 iodine atoms, T3 has 3). Seaweed, iodized salt, seafood. Caution:Excess iodine (>400 mcg) can worsen Hashimoto's in susceptible individuals. | Selenium (200 mcg/day):Critical for thyroid hormone synthesis and T4→T3 conversion. Also reduces TPO antibodies in Hashimoto's. Brazil nuts (2-3/day), seafood, organ meats. | Zinc (15-30 mg/day):Required for thyroid hormone production and conversion. Oysters, red meat, pumpkin seeds. | Iron (ferritin >70 ng/mL):Iron deficiency impairs thyroid peroxidase enzyme, worsening hypothyroidism. Check ferritin;supplement if low. | Vitamin D (>40 ng/mL optimal):Deficiency linked to higher risk of Hashimoto's and hypothyroidism.

Address Hashimoto's Thyroiditis (if TPO antibodies elevated)

5. Hashimoto's is the most common cause of hypothyroidism (90% of cases in iodine-sufficient countries). It's an autoimmune attack on the thyroid gland. | Selenium supplementation:200 mcg/day reduces TPO antibodies by ~40% in studies. | Gluten-free diet:Some Hashimoto's patients improve on gluten-free diet due to molecular mimicry between gliadin and thyroid tissue. | Stress management:Chronic stress exacerbates autoimmune conditions. Meditation, sleep, adaptogenic herbs may help.

Lifestyle Factors

Symptoms of Abnormal TSH (Thyroid Stimulating Hormone)

Low TSH (Thyroid Stimulating Hormone)

Avoid goitrogens in excess:Raw cruciferous vegetables (broccoli, kale, cauliflower) contain goitrogens that can interfere with iodine uptake. Cooking inactivates them. Not a problem unless iodine-deficient or eating massive amounts raw.
Avoid soy in large amounts:Soy isoflavones may interfere with thyroid hormone synthesis if iodine intake is marginal.
Timing of levothyroxine:Take 30-60 minutes before breakfast on empty stomach. Avoid calcium, iron supplements, coffee within 4 hours (impair absorption).

High TSH (Thyroid Stimulating Hormone)

Low TSH indicates overactive thyroid. Check Free T4/T3 (will be elevated). Causes include Graves'disease, toxic nodular goiter, thyroiditis.
Treatment options:Antithyroid medications (methimazole, PTU), radioactive iodine ablation, or surgery. Work with endocrinologist.

Causes of Abnormal TSH (Thyroid Stimulating Hormone)

Low Levels

High TSH=hypothyroidism (underactive thyroid). These are the symptoms when TSH is elevated.

High Levels

Rapid or irregular heartbeat (palpitations, atrial fibrillation)
Anxiety, nervousness, irritability
Tremors (especially hands)
Unexplained weight loss despite normal or increased appetite
Heat intolerance, excessive sweating
Insomnia, difficulty sleeping
Frequent bowel movements or diarrhea
Muscle weakness (especially in upper arms and thighs)
Eye problems (bulging eyes in Graves'disease)
Light or absent menstrual periods
Increased energy initially, followed by fatigue and burnout

When to Retest

Low TSH=hyperthyroidism (overactive thyroid). These are the symptoms when TSH is suppressed.

Scientific Evidence

Hashimoto's thyroiditis:Autoimmune attack on thyroid gland. Most common cause of hypothyroidism in iodine-sufficient countries (5-10% prevalence, 7:1 female:male ratio). Check TPO antibodies.|Iodine deficiency:Rare in developed countries (iodized salt) but most common cause worldwide. Thyroid can't make T4/T3 without iodine.|Thyroid surgery or radioactive iodine treatment:Surgical removal or ablation of thyroid gland for cancer, nodules, or hyperthyroidism.|Medications:Lithium (bipolar disorder), amiodarone (heart arrhythmias), high-dose iodine, interferon.|Pituitary or hypothalamus disorders:Rare. Tumor, trauma, or Sheehan's syndrome (postpartum pituitary necrosis) causes low TSH despite hypothyroidism ("central hypothyroidism").|Congenital hypothyroidism:Born with absent or malfunctioning thyroid gland. Screened at birth in developed countries.

Graves'disease:Autoimmune condition where antibodies stimulate thyroid to overproduce hormone. Most common cause of hyperthyroidism. Associated with bulging eyes (Graves'ophthalmopathy).|Toxic nodular goiter:Thyroid nodules autonomously produce excess thyroid hormone independent of TSH control.|Thyroiditis (inflammation):Subacute, postpartum, or silent thyroiditis causes stored thyroid hormone to leak out, temporarily suppressing TSH. Usually self-limited.|Excessive thyroid hormone medication:Over-replacement with levothyroxine or taking thyroid hormone without medical supervision (weight loss abuse).|Pituitary adenoma (rare):TSH-secreting tumor causes high thyroid hormones + high TSH (opposite of usual pattern).

Source:If starting or adjusting thyroid medication:Retest TSH, Free T4, Free T3 after 6-8 weeks of any dose change.|Once stable on treatment:Retest every 6-12 months to ensure dose remains appropriate.|If subclinical hypothyroidism (TSH 4-10):Retest in 3-6 months to see if it progresses. Check TPO antibodies to assess autoimmune risk.|If optimizing for longevity/symptoms:Retest 3 months after implementing iodine, selenium, or lifestyle changes to assess impact.|Routine screening:Every 5 years starting at age 35, or sooner if family history, autoimmune disease, or symptoms.

TSH as Screening Test

TSH is the most sensitive single test for primary hypothyroidism and hyperthyroidism. A normal TSH has 98% negative predictive value for excluding thyroid dysfunction in ambulatory patients. However, TSH may be normal in central hypothyroidism (pituitary failure) and doesn't reflect tissue thyroid status in T4→T3 conversion disorders.

Source:Garber JR, et al. Clinical practice guidelines for hypothyroidism in adults. Thyroid. 2012;22(12):1200-1235.

Optimal TSH Range Controversy

While standard lab range is 0.4-4.0 mIU/L, population studies show 95% of healthy individuals without thyroid disease have TSH <2.5 mIU/L. The upper limit of 4.0 includes many people with subclinical hypothyroidism. Many functional medicine practitioners target 0.5-2.5 mIU/L for optimal metabolic function.

Source:Wartofsky L, Dickey RA. The evidence for a narrower thyrotropin reference range is compelling. J Clin Endocrinol Metab. 2005;90(9):5483-5488.

Hashimoto's Thyroiditis Prevalence

Hashimoto's is the most common cause of hypothyroidism in iodine-sufficient countries, affecting 5-10% of the population with a 7:1 female:male ratio. It's characterized by elevated TPO and/or thyroglobulin antibodies. Selenium supplementation (200 mcg/day) reduces TPO antibodies by ~40% in multiple trials.

Source:Chiovato L, et al. Hypothyroidism in Context:Where We've Been and Where We're Going. Adv Ther. 2019;36(Suppl 2):47-58.

Subclinical Hypothyroidism Treatment Debate

Subclinical hypothyroidism (TSH 4-10 with normal Free T4) affects 4-10% of adults. Treatment remains controversial. Evidence supports treatment if:TSH >10, elevated TPO antibodies, symptomatic, pregnant/trying to conceive, or progressive TSH rise. For TSH 4-7 without symptoms, watchful waiting may be appropriate.

Source:Bekkering GE, et al. Thyroid hormones treatment for subclinical hypothyroidism:a clinical practice guideline. BMJ. 2019;365:l2006.

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