This may be mildly elevated in mild or subclinical hypothyroidism
TSH levels may be suppressed in hyperthyroidism.
The anterior pituitary is very sensitive to minor changes in thyroid hormone levels which is why TSH is usually considered to be the most reliable investigation of thyroid function.
2- Plasma free T4- confirms the diagnosis and assesses the severity of hyperthyroidism. It is also used to diagnose secondary hypothyroidism.
Both TSH and free T3 and T4 levels should be considered as it may be misleading in patients suffering from any pituitary disease.
Eg: TSH may be normal or low in secondary hypothyroidism
Raised T4 and T3 levels may be seen in initial stage of levothyroxine therapy.
Other investigations may include measurement of antibodies against TSH receptors or other Thyroid antigens
For instance: Antibodies to TPO, Thyroglobulin, TSH Receptor.
Some drugs may affect TFTs. For example: amiodarone may cause a decrease in free T4 and elevation of TSH. Amiodarone may also cause a decrease in TSH causing hyperthyroidism in certain patients.
-Rudrani