Thyroid Function tests

Question 1

What is the classic TFT pattern in thyrotoxicosis (e.g. Graves’ disease)?

Answer 1

Low TSH with high free T4

Question 2

How do thyroid function tests appear in T3 thyrotoxicosis?

Answer 2

Low TSH with normal free T4 and raised T3

Question 3

What TFT pattern defines primary hypothyroidism?

Answer 3

High TSH with low free T4

Question 4

Why is TSH raised in primary hypothyroidism?

Answer 4

Loss of negative feedback due to low thyroid hormone levels

Question 5

What TFT pattern is seen in secondary hypothyroidism?

Answer 5

Low TSH with low free T4

Question 6

Why must steroid replacement be given before thyroxine in secondary hypothyroidism?

Answer 6

To prevent precipitating an adrenal crisis due to coexisting ACTH deficiency

Question 7

What is the typical TFT pattern in sick euthyroid syndrome (non-thyroidal illness)?

Answer 7

Low free T4 with low or inappropriately normal TSH

Question 8

Which thyroid hormone is particularly low in sick euthyroid syndrome?

Answer 8

T3

Question 9

In which clinical setting is sick euthyroid syndrome most commonly seen?

Answer 9

Hospital inpatients with acute or severe systemic illness

Question 10

Why is treatment usually not required in sick euthyroid syndrome?

Answer 10

Thyroid function normalises once the underlying illness resolves

Question 11

What TFT pattern defines subclinical hypothyroidism?

Answer 11

High TSH with normal free T4

Question 12

Why is TSH abnormal before free T4 in early thyroid disease?

Answer 12

TSH is a more sensitive marker of thyroid dysfunction

Question 13

What TFT pattern suggests poor compliance with levothyroxine?

Answer 13

High TSH with normal free T4

Question 14

Why can free T4 be normal in patients poorly compliant with thyroxine?

Answer 14

Patients may take levothyroxine shortly before blood tests, normalising T4 while TSH lags behind

Question 15

What effect does steroid therapy have on thyroid function tests?

Answer 15

Low TSH with normal free T4

Question 16

Why do steroids suppress TSH levels?

Answer 16

They inhibit TSH secretion at the pituitary level

Question 17

What key pitfall should be remembered when interpreting TFTs in illness?

Answer 17

Many causes of hypothyroidism may have an initial thyrotoxic phase