Thyroid Gland function tests:
Thyroid gland function tests are mentioned below:
- Hormone profile
- Basic Test
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:
- Hyperthyroidism suspected
- Hypothyroidism suspected or monitoring replacement
- Sick euthyroidism
- 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).
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:
- rapid growth
- compression signs
- dominant nodule on scintigraphy
- nodule ⥸3cm