Thyroid Physiology & Function Tests

Question 1

Outline the Hypothalamus-Pituitary-Thyroid axis

Functions & mechanism of T3/T4

How it is transported

Answer 1

1. Hypothalamus
   
   • Thyrotropin-releasing hormone (TRH)
2. Ant. Pituitary
   
   • Thyroid-stimulating hormone (TSH)
3. Thyroid
   
   • T4 (thyroxine) ⇒ T3 (triiodothyronine)

• T4/T3
  
  • -ve feedback on Ant. Pituitary ⇒ TSH
  • ⇒ nuclear receptors ⇒ Inc cell metabolism
  • ⇒ Bing to Thyroxine-Binding Globulin (TBG) to travel

Question 2

Outline the suspected pathology in the following Thyroid function test (TFT) results

1. Inc TSH Low T4
2. Inc TSH Normal T4
3. Inc TSH High T4
4. Low TSH High T3 or T4
5. Low TSH Normal T3 or T4
6. Low TSH Low T4
7. Low TSH Low T4 & T4

Answer 2

1. Inc TSH Low T4 - hypothyroidism
2. Inc TSH Normal T4 - treated/ subclinical hypothyroidism
3. Inc TSH High T4 - TSH secreting tumour or thyroid hormone resistence
4. Low TSH High T3 or T4 - hyperthyroidism
5. Low TSH Normal T3 or T4 - subclinical hyperthyroidism
6. Low TSH Low T4 - central hypothyroidism
7. Low TSH Low T4 & T4 - sick euthyroidism (systemic illness) or pituitary disorder

Question 3

What TFTs would you find in treated/ subclinical hypothyroidism?

Answer 3

High TSH, normal T4

Question 4

What TFTs would you find in central hypothyroidism?

Answer 4

Low TSH, Low T4

Question 5

What TFTs would you find in hyperthyroidism?

Answer 5

Low TSH, High T3 or T4

Question 6

What TFTs would you find in TSH secreting tumour or thyroid hormone resistance?

Answer 6

High TSH, High T4

Question 7

What TFTs would you find in hypothyroidism?

Answer 7

High TSH, Low T4

Question 8

What TFTs would you find in subclinical hyperthyroidism?

Answer 8

Low TSH, Normal T3 or T4

Question 9

What TFTs would you find in sick euthryroidism?

Answer 9

Low TSH, Low T3 & T4

Low everything! Systemic illness