Which thyroid hormones are measured routinely and what is their clinical utility?
- A measure of plasma TSH which is produced by the pituitary gland in response to thyrotropin-releasing hormone and stimulates production and secretion of thyroid hormones
- Usually considered the best initial test in the diagnosis of thyroid disease:
- Elevated levels can suggest hypothyroidism
- Decreased levels suggest hyperthyroidism
- Serial measurements can be used to manage thyroid hormone therapy
- A measure of the metabolically active quantity of thyroxine, which circulates unbound to serum proteins
- Used to determine thyroid over or underactivity in the setting of an abnormal TSH;
- Elevated levels can confirm thyrotoxicosis
- Decreased levels can confirm hypothyroidism
- A measure of total T4 in the plasma, including both and unbound forms
- Abnormal levels despite normal thyroid function can be caused by conditions which alter levels of thyroid binding globulin:
- Drugs or illness can increase the thyroid biding proteins available to carry T4 and increase total levels despite a constant absolute concentration of free "active hormone"
- An example is pregnancy or hyper oestrogenic states which increase levels of TBG and thus increase levels of total T4
- A measure of the metabolically active quantity of T3:
- The major source of T3 comes from peripheral conversion of T4
- Can be used to confirm hyperthyroidism:
- Hyperthyroidism usually associated with greater increase in T3 than T4
- Small proportion of patients with hyperthyroidism have a normal T4 but an elevated T3 (so called "T3 toxicosis")
- Conversely many patients with hypothyroidism have normal T3 levels
- T3 levels are often decreased in clinically euthyroid patients with non-thyroidal illness (so called "sick-euthyroid syndrome"
- Difficulty in interpreting abnormal results makes it a poor initial screening test
What is the initial screening test of choice for diagnosis of thyroid dysfunction?
- TSH traditionally recommended as first-line screening as levels often change markedly in response to small variations in thyroid hormones
- Screening with TSH values alone can lead to misdiagnosis:
- TSH values may be normal despite significant hypothalamic-pituitary dysfunction meaning conditions are missed
- Abnormal levels do not differentiate between clinical and sub-clinical disease
- Most laboratories use combined TSH and fT4 levels as an initial screening test
Which autoantibodies are useful in the diagnosis of thyroid disease?
Thyroid peroxidase antibody
TSH Receptor Stimulating Antibody
TSH Receptor Blocking Antibody
Which diseases are the thyroid autoantibodies associated with?
- Autoimmune thyroid disease represents a spectrum of disease
- Each antibody can be associated with a range of clinical conditions
What are the common hormone abnormalities seen in thyroid disease?
Sick Euthyroid syndrome
Recovery from Sick Euthyroid syndrome
What is meant by subclinical disease?
- Thyroid dysfunction is reflected by abnormal TSH levels
- Reference ranges for thyroid hormones are wide and changes significant enough to render a patient hypo- or hyperthyroid may still be numerically within normal limits (sub-clinical disease)
Which TFT patterns can be seen in non-thyroidal illness?
- Three patterns of change in thyroid hormone levels have been described in non-thyroidal illness:
(Sick euthyroid syndrome)
Which abnormal TFT patterns can be seen with commonly used drugs?
- Common during the first few months due to inhibition of DIO preventing peripheral conversion of T4 to T3
- Thyrotoxicosis can occur due to:
- Large iodine load in drug precipitating thyroid autonomy
- Destructive drug induced thyroiditis
- Hypothyroidism due to failure to escape from Wolff-Chaikoff effect
- Overt or subclinical hypothyroidism
- Drug induced thyroiditis
- Primary hypothyroidism due to direct toxic effect
- Prodromal thyrotoxic phase may be seen