Thyroid Gland Physiology & Thyroid function test

Thyroid Gland function tests:

Thyroid gland function tests are mentioned below:

  • Hormone profile
  • Screening
  • Basic Test
  • Physiology

Thyroid gland physiology:

Thyroid-stimulating hormone (TSH=thyro tropin), a glycoprotein, is produced from the anterior pituitary. Thyroid gland produces mainly T4, which is 5-fold less active than T3. 85% of T3 is formed from peripheral conversion of T4. Most T3 and T4 in plasma is protein bound, eg to thyroxine-binding globulin (TBG). The unbound portion is the active part. T3 and T4  ⇡cell metabolism, via nuclear receptors, and are thus vital for growth and mental development. They also ⇡catecholamine effects. Thyroid hormone abnormalities are usually due to problems in gland itself and Rarely caused by the hypothalamus or the anterior pituitary.

Thyroid gland basic test:

Free T4 and T3 are more useful than total T4 and T3 as the latter are affected by TBG. Total T4 and T3 are ⇡ when TBG is ⇡ and vice versa. TBG is ⇡ in pregnancy, oestrogen therapy (HRT, oral contraceptives), and hepatitis. TBG is ⇣ in nephrotic syndrome and malnutrition (both from protein loss), drugs (androgens, cortico steroids, phenytoin), chronic liver disease, and acromegaly. TSH is very useful:

  1.   Hyperthyroidism suspected
  2.   Hypothyroidism suspected or monitoring replacement 
  3.   Sick euthyroidism
  4.   Assay interference

• Hyperthyroidism suspected:

Ask for T3, T4, and TSH. All will have ⇣TSH (except the rare TSH-secreting pituitary adenoma). Most have ⇡T4, but ~1% have only raised T3.  TSH varies through the day: trough at 2PM; 30% higher during darkness, so during monitoring, try to do at the same time.

 • Assay interference: 

It is caused by antibodies in the serum, interfering with the test.

Other Thyroid function tests:

• Thyroid autoantibodies:

Antithyroid peroxidase (TPO; formerly called microsomal) antibodies or antithyroglobulin antibodies may be increased in autoimmune thyroid disease: Hashimoto’s or Graves’ disease. If +ve in Graves’, there is an increased risk of developing hypothyroidism at a later stage. 

 TSH receptor antibody: 

May be ⇡ in Graves’ disease (useful in pregnancy). 

• Ultrasound: 

If a solitary (or dominant) large nodule, in a multi nodular goitre, do a fine-needle aspiration to look for thyroid cancer.

• Isotope scan: 

(123 Iodine, 99 technetium pertechnetate, etc;  Useful for determining the cause of hyperthyroidism and to detect retrosternal goitre, ectopic thyroid tissue or  metastases (+ whole body CT). If there are suspicious nodules, the question is: does the area have increased (hot), decreased (cold), or the same (neutral) uptake of isotope as the remaining thyroid. Surgery is most likely to be needed if: 

  1. rapid growth 
  2. compression signs 
  3. dominant nodule on scintigraphy 
  4. nodule ⥸3cm 
  5. hypo-echogenicity. 

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