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Understanding Thyroid Function Tests

Apr 19, 2025

Thyroid Function Tests Lecture

Learning Objectives

  • Describe the general structure and function of hormones in thyroid gland regulation.
  • Diagram the relationship between thyroid hormones.
  • Use thyroid function tests (TFTs) to diagnose four general categories of thyroidal illness.
  • List non-thyroidal conditions which can affect TFTs.

Overview of Thyroid Function Tests (TFTs)

  • TFTs interpretation seems easy but can be confusing, especially with non-conforming tests.
  • Biosynthesis of thyroid hormone is complex; focus on key aspects.
  • Terminology can be confusing with four hormones involved in the process.

Thyroid Hormones

Thyrotropin Releasing Hormone (TRH)

  • A tripeptide amide formed in the hypothalamus.
  • Stimulates release of TSH in the anterior pituitary.

Thyroid Stimulating Hormone (TSH)

  • Known as thyrotropin, a glycoprotein.
  • Increases physiologic actions of thyroid hormone, including release, formation, and thyroid cell growth.

True Thyroid Hormones (T3 and T4)

  • Triiodothyronine (T3) and Thyroxine (T4) are synthesized and released by the thyroid gland.
  • Require tyrosine and iodine.
  • Stored in the thyroid as thyroglobulin.
  • Ratio: 90% T4 to 10% T3.
  • Mainly bound to plasma proteins; free form is active.

Biological Activity

  • T3 is more biologically active than T4.
  • Tissues convert T4 to active T3 and inactive reverse T3 (rT3).
  • Functions include increasing basal metabolic rate, metabolism, bone maturation, and cardiac output.

Regulation of Thyroid Hormones

  • Begins at the hypothalamus with TRH release, stimulates TSH, which then stimulates T4 formation.
  • T4 converted to T3 in peripheral tissues.
  • Negative feedback mechanisms regulate hormone levels.
  • The Wolf-Chaikoff and Jod-Basedow effects describe responses to iodine load.

Thyroid Function Tests (TFTs)

Common Tests

  • TSH, Free T4, and sometimes Free T3.
  • Total T4, T3, and reverse T3 tests less common.

Interpretation

  • High TSH + low hormones: Primary hypothyroidism.
  • Low TSH + high hormones: Primary hyperthyroidism.
  • Low TSH + low hormones: Central hypothyroidism or non-thyroidal illness.
  • High TSH + high hormones: Secondary hyperthyroidism or rare disorders.
  • Subclinical hypothyroidism/hyperthyroidism defined by abnormal TSH with normal hormones.

Non-Thyroidal Conditions Affecting TFTs

  • U thyroid 6 syndrome can alter TFTs due to non-thyroidal illness.
  • Common causes include pregnancy, illness, liver/renal disease, malnutrition, medications.

Ordering TFTs

  • TSH, Free T4, and Free T3 as basic tests unless unusual circumstances.
  • Consider deferring TFTs in acutely ill patients.
  • Follow specific steps based on suspicion for hypo/hyperthyroidism.

Anti-Thyroid Antibodies

  • Anti-thyroglobulin, anti-thyroid peroxidase: Seen in Hashimoto's thyroiditis.
  • Anti-TSH receptor antibodies: Graves' disease and Hashimoto's.
  • Antibody measurement can aid in diagnosis.

This concludes the lecture on thyroid function tests. Remember to review the interplay between hormones and conditions that can affect TFTs for a comprehensive understanding.

Full transcript