Thyroid Function: TSH, T3, and T4 Explained
Your thyroid regulates metabolism, energy, and body temperature. Learn how to interpret thyroid tests and recognize when 'normal' might not be optimal.
Reference: TSH: 0.4-4.0 mIU/L, Free T4: 0.8-1.8 ng/dL mIU/L (TSH), ng/dL (T4), pg/mL (T3)
Key Takeaway
Thyroid function affects every cell in your body. Many people with thyroid symptoms have "normal" TSH but may still have subclinical thyroid dysfunction. Understanding the full thyroid panel and interpreting results in clinical context is essential.
What is the Thyroid?
Your thyroid is a butterfly-shaped gland in your neck that produces hormones controlling:
- Metabolic rate - How fast you burn calories and produce energy
- Body temperature - Regulation and heat production
- Heart rate - Speed and force of heartbeat
- Digestion - GI motility and function
- Mood and cognition - Brain function and mental health
- Skin, hair, and nails - Growth and repair
- Reproductive function - Fertility and menstrual cycles
The Thyroid Hierarchy
The thyroid operates under a feedback loop:
- Hypothalamus releases TRH (thyrotropin-releasing hormone)
- Pituitary responds by releasing TSH (thyroid-stimulating hormone)
- Thyroid responds to TSH by producing T4 (mostly) and T3 (active)
- T4 converts to T3 in tissues throughout the body
- T3 provides feedback - High T3 lowers TSH, low T3 raises TSH
This is why TSH is elevated in hypothyroidism—the pituitary is screaming at the thyroid to work harder.
Understanding Your Thyroid Panel
| Test | What It Measures | High Means | Low Means |
|---|---|---|---|
| TSH | Pituitary signal to thyroid | Thyroid UNDERACTIVE | Thyroid OVERACTIVE |
| Free T4 | Active thyroid hormone (storage form) | Hyperthyroidism or thyroiditis | Hypothyroidism |
| Free T3 | Most active thyroid hormone | Hyperthyroidism | Hypothyroidism or poor T4-T3 conversion |
| Reverse T3 | Inactive T3 (stress hormone) | High stress, chronic illness | Rarely measured directly |
| Source: Clinical guidelines and literature review | |||
Understanding Your Results
Reference Range
The 'Normal' Range Problem
The TSH reference range (0.4-4.0 mIU/L) includes people with thyroid disease. Many healthy people have TSH ＜ 2.0. Levels above 2.5 may indicate early thyroid dysfunction even though they're still within the "normal" lab range. Functional medicine practitioners often recommend TSH be kept below 2.5 for optimal health.
What Your Levels Mean
Symptoms of Thyroid Dysfunction
Recognizing Thyroid Problems
| Factor | Effect | What to Do |
|---|
Always tell your doctor about medications, supplements, and recent health events before testing.
Causes of Thyroid Dysfunction
Hypothyroidism Causes
🔬Why Thyroids Slow Down
Hashimoto's thyroiditis (autoimmune) accounts for 90% of hypothyroidism in developed countries:
- Your immune system attacks thyroid tissue
- More common in women (7:1 female:male ratio)
- Associated with other autoimmune diseases
- TPO antibodies confirm the diagnosis
Other causes:
- Iodine deficiency - Rare in countries with iodized salt
- Thyroid surgery or radiation - For cancer, nodules, or Graves' disease
- Medications - Lithium, amiodarone, some cancer treatments
- Pituitary problems - Rare; TSH is low instead of high
- Pregnancy - Can trigger thyroid problems
Hyperthyroidism Causes
- Graves' disease - Autoimmune overproduction of thyroid hormone
- Toxic nodules - Hot nodules producing hormone independently
- Thyroiditis - Temporary inflammation causing hormone leakage
- Excessive thyroid medication - Overtreatment of hypothyroidism
Testing Recommendations
Basic panel:
- TSH (primary screening test)
Complete panel:
- TSH
- Free T4 (active, available form)
- Free T3 (most active form)
When to test further:
- TPO antibodies - If autoimmune thyroiditis suspected
- Reverse T3 - If conversion problems suspected
- Thyroid ultrasound - If nodules or enlargement detected
Timing: Test in the morning, fasting if possible. TSH has a diurnal rhythm—highest in early morning. Some medications affect results; tell your doctor about all supplements.
Nutrients for Thyroid Health
| Nutrient | Role in Thyroid Function | Daily Target |
|---|---|---|
| Selenium | Converts T4 to active T3 | 200 mcg |
| Iodine | Building block of thyroid hormones | 150 mcg (not more without testing) |
| Zinc | T4 to T3 conversion, hormone synthesis | 15-30 mg |
| Iron | Needed for T4 to T3 conversion | Adequate ferritin > 50 ng/mL |
| Vitamin D | Modulates immune function | 40-60 ng/mL |
| Tyrosine | Amino acid for hormone production | Adequate protein intake |
| Source: Clinical guidelines and literature review | ||
Iodine Caution
While iodine is essential for thyroid function, EXCESS iodine can trigger thyroid problems in susceptible people. If you have Hashimoto's or nodular disease, high-dose iodine supplements (like kelp) may worsen your condition. Get iodine from iodized salt and food, not concentrated supplements, unless prescribed.
Related Biomarkers
🔗Related Biomarkers
Treatment Approaches
Hypothyroidism Treatment
Levothyroxine (T4 only):
- Standard treatment, most common
- Converts to T3 in body (requires healthy conversion)
- Brands: Synthroid, Levoxyl, Tirosint
- Take on empty stomach, 30-60 min before breakfast
Liothyronine (T3):
- Fast-acting, used in combination therapy
- Brands: Cytomel
- Short half-life, requires twice-daily dosing
Natural desiccated thyroid:
- Contains T4, T3, and other thyroid compounds
- Brands: Nature-throid, NP Thyroid, Armour
- Some patients prefer this approach
Goal: TSH in optimal range (0.5-2.5) with symptom resolution.
Hyperthyroidism Treatment
- Antithyroid medications - Methimazole, PTU
- Radioactive iodine - Destroys overactive thyroid tissue
- Surgery - Thyroidectomy (removal)
- Beta blockers - Control symptoms (rapid heart rate, anxiety)
Frequently Asked Questions
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