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In the primary care setting, thyroid diseases are almost never diagnosed per se.

The commonly utilized thyroid function test which run serum assays of TSH (thyroid stimulating hormone) serves to measure thyroid function (not detect the disease); and usually treatment is initiated based on the interpretation of the deviation of the assay from its normal value range.

Ideally, the TSH is ordered after a comprehensive history and thorough physical examination that points the physician to a considerable high index of suspicion. In most centers however, the thyroid function test is run routinely. Below is a rough guide to interpreting the thyroid function test:

Suspected hypothyroidism:
Normal TSH, normal T4 -> euthyroid (everything's fine!)
High TSH, low T4-> primary hypothyroidism (Hashimoto's thyroiditis)
Normal or low TSH, low T4 -> secondary hypothyroidism (pituitary failure)
High TSH, high T4-> thyroid hormone resistance (rare)

Suspected hyperthyroidism:
Low TSH, high T4 -> hyperthyroidism (Graves' disease)
Low TSH, normal T4-> inconclusive; obtain T3. If high-> T3 toxicosis. If normal-> subclinical hyperthyroidism
Normal or high TSH, high T4 -> secondary hyperthyroidism(TSH resistance or TSH secreting pituitary tumor)

As mentioned, TSH or thyroid hormone level deviation from the normal only indicates a disease process, (hypo or hyperthyroidism); not the disease itself. Thyroid diseases such as Hashimoto's, Graves, Toxic Nodular Goiter, Thyroid Resistance, neoplasms which can produce either hyper, hypo-thyroid or even euthyroid states can only be diagnosed on histology with tissue samples obtained from fine-needle biopsy; or an imaging modality (ultrasound/ MRI)

Such definite diagnosis is only indicated when patients do not respond or respond suboptimally to treatment; or they are suspected to have a protracted disease course (i.e. cancers).

Therefore, if you are on Synthroid or Propylthiouracil without having had a biopsy; you are being treated empirically (without clear evidence of disease). This is acceptable practice as long as you undergo thyroid function tests routinely and have your medications adjusted on a regular basis.

Learn more about this author, Ernie Yap.
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