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Thyroid Function


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 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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