Thyroid Panel Explained: TSH, T3, T4, and What Your Results Mean

Your thyroid is a small, butterfly-shaped gland at the base of your neck, and it has an outsized influence on how your entire body functions. It controls your metabolism, energy production, body temperature, heart rate, menstrual cycles, and even how quickly your hair grows. When it's off, you feel it everywhere.

Thyroid disorders are remarkably common — affecting roughly 20 million Americans, with women being five to eight times more likely to be affected than men. Yet thyroid issues are frequently underdiagnosed or inadequately tested. Here's what each thyroid test measures and what your results actually mean.

The Thyroid Feedback Loop

Before diving into the numbers, it helps to understand how the system works.

Your thyroid operates on a feedback loop controlled by your brain:

1. The hypothalamus (in your brain) releases TRH (thyrotropin-releasing hormone)
2. The pituitary gland responds by releasing TSH (thyroid-stimulating hormone)
3. The thyroid gland responds to TSH by producing T4 and T3
4. When T3 and T4 levels are adequate, the brain reduces TSH production (negative feedback)

Think of it like a thermostat. TSH is the thermostat setting, and T3/T4 are the temperature. When the house is cold (low thyroid hormone), the thermostat turns up (high TSH). When the house is warm enough (adequate thyroid hormone), the thermostat turns down (low TSH).

This relationship is why TSH and thyroid hormones typically move in opposite directions.

TSH (Thyroid-Stimulating Hormone)

What it measures: TSH is produced by the pituitary gland and tells your thyroid how hard to work. It's the most sensitive initial screening test for thyroid function.

Normal range: Approximately 0.4 to 4.0 mIU/L (some labs use 0.5 to 4.5)

What high TSH means:

• Your pituitary is shouting at a thyroid that isn't producing enough hormone
• This indicates hypothyroidism — an underactive thyroid
• The higher the TSH, generally the more underactive the thyroid

What low TSH means:

• Your pituitary is whispering because there's already too much thyroid hormone in your system
• This indicates hyperthyroidism — an overactive thyroid
• Or you're taking thyroid medication at too high a dose

The controversy about "normal": There's ongoing debate in endocrinology about the upper limit of normal TSH. Some experts argue that a TSH above 2.5 may indicate early thyroid decline, particularly in women with symptoms. The standard lab range of up to 4.0-4.5 includes some people with subclinical hypothyroidism. This is why symptoms matter alongside the number.

Free T4 (Thyroxine)

What it measures: T4 is the primary hormone your thyroid produces. The "free" designation means it measures only the unbound, active portion — not the T4 attached to carrier proteins.

Normal range: Approximately 0.8 to 1.8 ng/dL

What to know: T4 is the storage form of thyroid hormone. Your body produces about 80-90% of its thyroid hormone as T4, which then gets converted to T3 (the active form) in your tissues — primarily in the liver and kidneys.

Low free T4 with high TSH confirms hypothyroidism. Low free T4 with normal or low TSH might indicate a pituitary problem rather than a thyroid problem.

Free T3 (Triiodothyronine)

What it measures: T3 is the biologically active thyroid hormone — the one that actually drives your metabolism, energy, and cellular function. It's about four times more potent than T4.

Normal range: Approximately 2.3 to 4.2 pg/mL

Why it matters: Some patients have normal TSH and normal T4 but low T3. This can happen when the conversion of T4 to T3 is impaired — a situation caused by stress, inflammation, caloric restriction, selenium or zinc deficiency, or chronic illness. These patients may have symptoms of hypothyroidism despite "normal" standard labs.

This is why a complete thyroid panel — not just TSH alone — gives a fuller picture.

Hypothyroidism: The Underactive Thyroid

Hypothyroidism means your thyroid isn't producing enough hormone. The most common cause in the United States is Hashimoto's thyroiditis, an autoimmune condition where your immune system attacks the thyroid gland.

Common symptoms:

• Fatigue that sleep doesn't fix
• Unexplained weight gain or difficulty losing weight
• Feeling cold when others are comfortable
• Constipation
• Dry skin and brittle nails
• Hair thinning or loss (diffuse pattern, not patches)
• Brain fog and poor concentration
• Depression and low mood
• Menstrual irregularities (heavier or irregular periods)
• Puffy face and swollen hands/feet
• Elevated cholesterol

Lab pattern: High TSH, low free T4, possibly low free T3

The connection between thyroid function and weight is real but often overstated. Hypothyroidism causes metabolic slowing that can contribute 5 to 15 pounds of weight gain, mostly from water retention. It doesn't usually cause dramatic obesity on its own — but it makes weight loss significantly harder, which matters for patients pursuing weight management.

Hyperthyroidism: The Overactive Thyroid

Hyperthyroidism means your thyroid is overproducing hormone. The most common cause is Graves' disease, another autoimmune condition.

Common symptoms:

• Unexplained weight loss despite normal or increased appetite
• Rapid or irregular heartbeat (palpitations)
• Anxiety, nervousness, irritability
• Tremor in hands and fingers
• Excessive sweating and heat intolerance
• More frequent bowel movements
• Thinning skin and fine, brittle hair
• Difficulty sleeping
• Menstrual changes (lighter or absent periods)
• Bulging eyes (in Graves' disease specifically)

Lab pattern: Low TSH, high free T4, possibly high free T3

Hyperthyroidism is less common than hypothyroidism but requires prompt treatment because it stresses the heart and can cause dangerous arrhythmias if left unchecked.

Thyroid and Hair Loss

Thyroid dysfunction is one of the most common medical causes of hair loss, and it's frequently the first thing to check when someone presents with diffuse thinning.

Both hypothyroidism and hyperthyroidism can cause hair loss, but they do it differently:

• Hypothyroidism typically causes diffuse thinning — your hair becomes thinner overall, including the outer third of the eyebrows. The hair may become dry, coarse, and brittle.
• Hyperthyroidism causes fine, fragile hair that breaks easily.

The good news: thyroid-related hair loss is usually reversible once thyroid levels are properly managed. It can take several months for regrowth to become visible, so patience is necessary.

Thyroid Antibodies

If your TSH or thyroid hormones are abnormal, your doctor may order thyroid antibodies:

• TPO antibodies (anti-thyroid peroxidase): Elevated in Hashimoto's thyroiditis
• Thyroglobulin antibodies: Also associated with Hashimoto's
• TSI (thyroid-stimulating immunoglobulin): Elevated in Graves' disease

These antibodies help identify whether your thyroid problem is autoimmune in origin, which affects treatment decisions and prognosis.

When "Normal" Labs Don't Feel Normal

Many patients with thyroid symptoms are told their labs are "normal" because their TSH falls within the broad reference range. But a TSH of 3.8 in a woman with fatigue, weight gain, hair loss, and constipation is a different clinical situation than a TSH of 3.8 in someone who feels perfectly fine.

This is where the conversation between doctor and patient matters. At Coral, we look at the complete picture: TSH, free T4, free T3, symptoms, and clinical context. We don't dismiss symptoms because a single number falls within a reference range.

How Coral Approaches Thyroid Testing

At Coral, thyroid function is part of our standard evaluation for patients presenting with weight concerns, hair loss, fatigue, and mood changes. We order a complete thyroid panel — not just TSH — and we correlate your labs with your symptoms before making recommendations.

If you've been told your thyroid is "fine" but you don't feel fine, it may be time for a closer look.

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