What do TSH, T3, and T4 mean?

TSH (Thyroid Stimulating Hormone) is produced by your pituitary gland and signals your thyroid to make hormones. T4 (thyroxine) is the inactive thyroid hormone, and T3 (triiodothyronine) is the active form your cells use. Together they regulate metabolism, energy, body temperature, and many other functions.

What is a normal TSH level?

Standard TSH reference range is 0.4 to 4.0 mIU/L. However, many endocrinologists consider 1.0 to 2.5 mIU/L optimal for most adults. Your specific target depends on age, pregnancy status, and whether you are on thyroid medication.

Why test free T3 and free T4?

Free T3 and free T4 measure the unbound, biologically active thyroid hormones your cells actually use. Total T3 and T4 include hormone bound to proteins in your blood, which is not active. Free hormone testing gives a more accurate picture of your thyroid status.

How often should thyroid be tested?

Healthy adults: every 2-3 years after age 35, or sooner with symptoms. On thyroid medication: every 6-8 weeks after dose change, then every 6-12 months once stable. Pregnancy: at confirmation, then every 4-6 weeks.

Thyroid Function Explained: TSH, T3, T4 Infographic

🦋 Your Thyroid Gland

Key Facts About Your Thyroid

Butterfly-shaped gland in your neck
Produces hormones that regulate metabolism
Controls energy, temperature, weight, mood, and more
When out of balance, affects nearly every body system

🔬 The Three Key Thyroid Hormones

TSH

Thyroid Stimulating Hormone

Made by: Pituitary gland (brain)
Function: Signals your thyroid to produce T4 and T3
Key Point: High TSH = Low thyroid function
Low TSH = High thyroid function

T4

T4 (Thyroxine)

The Storage Hormone

Made by: Thyroid gland
Function: Inactive form that converts to active T3
Key Point: Most abundant thyroid hormone (80%)
Must convert to T3 to be used

T3

T3 (Triiodothyronine)

The Active Hormone

Made by: Thyroid gland + conversion from T4
Function: The active form that powers your cells
Key Point: 3-4x more potent than T4
This is what actually works in your body

Pituitary

Releases TSH

→

Thyroid

Makes T4 & T3

→

Body Cells

Use T3 for energy

📋 Understanding Your Lab Results

Standard Lab Reference Ranges

Test	Lab "Normal" Range	Optimal Range	What It Means
TSH	0.4 - 4.5 mIU/L	1.0 - 2.0 mIU/L	Lower is better for most people
Free T4	0.8 - 1.8 ng/dL	1.0 - 1.5 ng/dL	Middle to upper range ideal
Free T3	2.3 - 4.2 pg/mL	3.0 - 4.0 pg/mL	Upper third of range is best
TPO Antibodies	< 35 IU/mL	< 10 IU/mL	Lower = less autoimmune activity
Reverse T3	9.2 - 24.1 ng/dL	< 15 ng/dL	Lower is better

How to Read Your Results

TSH above 2.5: May indicate early hypothyroidism, especially with symptoms
TSH below 0.4: May indicate hyperthyroidism or overmedication
Low T4 or T3 with high TSH: Classic hypothyroidism
High T4 or T3 with low TSH: Classic hyperthyroidism
Normal TSH but low T3: Possible conversion problem
Positive antibodies: Autoimmune thyroid disease (Hashimoto's or Graves')

⚠️ Recognizing Thyroid Problems

Hypothyroidism (Low)

Fatigue and low energy
Weight gain
Feeling cold
Dry skin and hair
Constipation
Brain fog
Depression
Heavy periods
Joint/muscle pain

Hyperthyroidism (High)

Anxiety and nervousness
Rapid heartbeat
Weight loss
Feeling hot/sweating
Trembling hands
Insomnia
Diarrhea
Light/missed periods
Irritability

🏥 When to Seek Treatment

1

You Experience Symptoms

Notice persistent symptoms like fatigue, weight changes, temperature sensitivity, or mood changes

2

Get Comprehensive Testing

Request full thyroid panel: TSH, Free T4, Free T3, TPO antibodies, and Reverse T3

3

Interpret Results Properly

Don't accept "normal" if you have symptoms. Look for optimal ranges, not just lab ranges

4

Find the Right Provider

Work with a doctor who treats symptoms + labs, not just labs alone

5

Start Treatment & Monitor

Begin appropriate treatment, retest in 6-8 weeks, adjust as needed until symptoms resolve

⚠️ Red Flags - Seek Immediate Care If You Experience:

Severe rapid heartbeat or chest pain
Severe anxiety or panic attacks
Difficulty breathing or swallowing
Sudden, severe fatigue or confusion
Rapid, unexplained weight loss (over 10% body weight)

💊 Treatment Options Overview

T4

Levothyroxine

Synthetic T4 Hormone

Most Common Medication
Works well for most people
Requires 6-8 weeks to see full effect
Regular monitoring needed

T3

Liothyronine

Synthetic T3 Hormone

Active Thyroid Hormone
Sometimes added to T4
Fast-acting
Helpful for conversion issues

NDT

Natural Desiccated

From Animal Thyroid

Contains T4, T3, T2, T1
Preferred by some patients
More natural hormone ratios
Requires careful monitoring

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Understanding Your Thyroid