Thyroid Function Tests Explained: TSH, Free T4, and Hashimoto’s Antibodies


 Introduction

Your thyroid gland is your body’s "thermostat." It controls your metabolism, energy levels, and even your mood. When this butterfly-shaped gland malfunctions, doctors order a Thyroid Panel

But looking at TSH, T3, and T4 levels can be confusing. As a Biochemist, I will explain how these hormones work together in a delicate "feedback loop" and what your results actually mean

 The "Thermostat" Mechanism (Negative Feedback Loop)

To understand the results, you must understand the relationship between the brain (Pituitary Gland) and the Thyroid

  • TSH (Thyroid Stimulating Hormone): This is the "messenger" sent by the Pituitary gland to tell the thyroid to work

  • :The Rule

    • High TSH: Means your thyroid is "lazy" (Low T3/T4). The brain is screaming (high TSH) to stimulate it. $\rightarrow$ Hypothyroidism

    • Low TSH: Means your thyroid is "overactive" (High T3/T4). The brain stops sending signals to prevent overproduction. Hyperthyroidism

 The Main Hormones: T4 and T3

The thyroid produces two main hormones. We measure them in two forms: Total and Free

  • Thyroxine (T4): The main hormone produced

    • Free T4 (FT4): This is the active form not bound to proteins. It is more accurate than Total T4 because it reflects what is actually available for your body to use

  • Triiodothyronine (T3): The stronger, more active hormone

    • Free T3 (FT3): Usually ordered when hyperthyroidism is suspected

The Autoimmune Detectives: Thyroid Antibodies

Sometimes, the thyroid malfunctions because the immune system attacks it. We use antibody tests to diagnose the cause

  • TPOAb (Thyroid Peroxidase Antibody): High levels usually indicate Hashimoto’s Thyroiditis, the most common cause of hypothyroidism

  • TGAb (Thyroglobulin Antibody): Another marker for autoimmune thyroid damage

  • TRAb / TSI: These antibodies stimulate the thyroid causing it to overwork. They are diagnostic for Graves’ Disease (Hyperthyroidism).

Tumor Markers & Monitoring

  • Thyroglobulin: A protein made by the thyroid. It is mainly used as a Tumor Marker after thyroid cancer surgery to monitor for recurrence

  • Calcitonin: Used specifically to screen for a rare type called Medullary Thyroid Cancer

Important Lab Instructions (Don't Ignore These!)

To get accurate numbers, follow these rules

  1. Timing: Early morning is best for TSH accuracy

  2. Medication: If you take thyroid replacement (e.g., Levothyroxine), take your dose AFTER the blood draw, not before

  3. The Biotin Warning: Supplements containing Biotin (Vitamin B7) (often used for hair and nails) can completely ruin the test results (causing false lows or highs). Stop taking Biotin 48 hours before your test

Conclusion

A thyroid profile is more than just TSH. By analyzing Free T4, T3, and antibodies, we get a full picture of your metabolic health. Always interpret these results in the context of your symptoms


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