Interpretation of Thyroid Function Tests
(Author : Dr. Sachin Sarpotdar)

Thyroid Function test are routinely advised in the suspected cases of Hypothyroidism and Hyperthyroidism. Hypothyroidism is more prevalent than hyperthyroidism. 
The patients of hypothyroidism usually present with weight gain problem, hair loss, low energy drive, amenorrhea, weakness, Cold intolerance, anemia and laziness.
In such cases Thyroid function test is recommended and level of TSH and FT4 are observed.
If there is high TSH and low FT4-   It is considered as the ‘Primary Hypothyroidism’. In this the pathology is present in the Thyroid gland itself.
The further probing is essential and usually Thyroid Microsomal Antibodies (Thyroid Peroxidase) are monitored.  Peroxidase is an enzyme which is essential in the conversion of Iodide into Iodine. The antibodies against this enzyme will naturally disturb the thyroid hormone synthesis. If these are observed in excess then it is diagnosed as ‘Hashimoto’s thyroiditis’.
If the level of these antibodies is normal then one can consider congenital deficiency of T4 or Iodine deficiency.
If there is high TSH and normal FT4-   It is then considered as Subclinical thyroiditis.
If there is Low TSH and Low FT4- It is then usually diagnosed as the secondary or the tertiary hypothyroidism. In secondary hypothyroidism the pathology is in pituitary gland and in the tertiary it is in the hypothalamus. For confirmation of the site TRH stimulation test is performed.  Low or no TSH response confirms the secondary hypothyroidism and the delayed response indicates the tertiary hypothyroidism.
Hyperthyroidism- The patients of hyperthyroidism usually appear with weight loss, increased pulse or heart rate, heat intolerance, tremors, dyspnea, anxiety, irritability, sweating, amenorrhea and infertility etc. In suspected cases of hyperthyroidism levels of TSH and FT4 are monitored.
TSH is low and high FT4-   It is then considered as the Primary hyperthyroidism. The further evaluation is done on the basis of TRAb  (Thyroid releasing antibody)  and Thyroid scan. TRAb mimics with TSH and it leads to enlargement of Thyroid gland in association with diffuse uptake of radioisotopes in ‘Graves Disease’
In toxic adenoma the TRAb is usually negative and  pattern of uptake in the scan is nodular whereas in multi nodular goiter the pattern is irregular.
Low TSH and Normal FT4-
 In this situation the FT3 is measured and its normal level conveys the diagnosis like subclinical or mild hyperthyroidism or non thyroid illness or the effects of drugs like amiodarone (anti arrhythmatic drug), steroid or a dopamine.
High T3 conveys T3 thyrotoxicosis.
High TSH and High FT4 convey the diagnosis of Pituitary secreting adenoma which is considered as secondary hyperthyroidism.
Thyrotropin releasing horomone (TRH)  test is carried out to confirm the resistance to thyroid hormone and adenoma. Increased response is observed in thyroid hormone resistance whereas no response is observed in adenoma.