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Thyroid Function
 
 

Overview

  • The TSH is a useful screening tool for thyroid disease. If the TSH is elevated or reduced, then the T4 is the next useful test for distinguishing the pattern of derangement.
  • Low TSHHigh TSH
    Low T4Central HypothyroidismPrimary hypothyroidism
    Normal T4Subclinical hyperthyroidismSubclinical hypothyroidism
    High T4Primary hyperthyroidismCentral hyperthyroidism

Suppressed TSH with Elevated T4

  • If the T4 is elevated and the TSH is appropriately suppressed, then primary hyperthyroidism is present.
  • Primary hyperthyroidism is hyperthyroidism that originates from thyroid tissue.
    • Causes of Primary Hyperthyroidism

    • Grave’s disease
    • Toxic multinodular goitre
    • Toxic nodule
    • Thyroiditis - postviral, postpartum, lymphocytic
    • Thyroxine - excess replacement, thyrotoxicosis factitia
    • Drugs - amiodarone, iodine
    • Pregnancy-related - hyperemesis gravidarum, hydatidiform mole
    • Struma ovarii
    • Congenital hyperthyroidism

Suppressed TSH with Normal T4

  • Reduced TSH with a normal T4 level is suggestive of subclinical hyperthyroidism, a condition which is often asymptomatic. There are several other causes that make up the differential diagnosis.
    • Causes of Reduced TSH with Normal T4

    • Subclinical hyperthyroidism
    • Recent treatment of hyperthyroidism
    • Intermittent thyroxine
    • T3 thyrotoxicosis
    • Nonthyroidal illness
    • Drugs - steroids, dopamine, dobutamine
    • Pearls

    • Ask about symptoms of hyperthyroidism.
    • If the patient is clinically thyrotoxic, consider T3 toxicosis and check the patient’s T3.
    • Take a careful medication history, including past treatment for hyper or hypothyroidism.
    • Ask about recent illnesses.

Reduced / Normal TSH with Reduced T4

  • A reduced or normal TSH with a reduced T4 level is classically suggestive of a central cause of hypothyroidism (i.e. reduced hypothalamic or pituitary release of TRH or TSH respectively). However, central hypothyroidism is rare and perhaps a more common cause is the non-thyroidal illness (sick euthyroid) syndrome.
    • Causes of Reduced / Normal TSH with Reduced T4

    • Nonthyroidal illness
    • Recent treatment of hyperthyroidism
    • Central hypothyroidism
    • Congenital TSH or TRH deficiency
    • Assay interference

Elevated TSH with Reduced T4

  • A reduced T4 with an appropriately suppressed TSH is indicative of primary hypothyroidism.
  • Primary hypothyroidism originates from pathology affecting thyroid tissue, resulting in reduced production of thyroid hormones.
    • Causes of Primary Hypothyroidism

    • Autoimmune thyroiditis - Hashimoto’s, atrophic
    • Hypothyroid phase of thyroiditis
    • Iatrogenic - radioiodine therapy, thyroidectomy, external irradiation of the neck
    • Drugs - amiodarone, lithium, interferons, interleukin-2, iodide
    • Iodine deficiency
    • Thyroid infiltration - amyloidosis, tumour
    • Congenital hypothyroidism

Elevated TSH with Normal T4

  • An elevated TSH with a normal T4 is suggestive of subclinical hypothyroidism, though may also occur in several other circumstances.
    • Causes of Elevated TSH with Normal T4

    • Subclinical hypothyroidism
    • Poor compliance with thyroxine
    • Assay interference
    • Drugs - amiodarone, sertraline
    • Nonthyroidal illness (recovery phase)
    • TSH receptor defects
    • TSH resistance
    • Pearls

    • Ask about symptoms of hypothyroidism
    • Take a careful medication history
    • Ask about recent illnesses

Normal / Elevated TSH with Elevated T4

  • A normal or elevated TSH with an elevated T4 is classically suggestive of central hyperthyroidism (a TSH-secreting tumour), however this is a rare disease and consideration should be given to other causes.
    • Causes of Normal / Elevated TSH with Elevated T4

    • TSH secreting pituitary tumour
    • Thyroid hormone resistance
    • Acute psychiatric illness
    • Drugs - amiodarone, heparin
    • Familial dysalbuminaemic hyperthyroxinaemia
    • Assay interference
 
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