Diagnosis of Hypothyroidism
Here we discuss the way in which hypothyroidism is diagnosed
Your Houston endocrinologist can diagnose hypothyroidism after he collects your medical history, examines your thyroid, reviews your medication list, and obtains specific blood tests which evaluate the hypothalamic-pituitary-thyroid axis. Use the thyroid glossary for terms that are unfamiliar. The most commonly ordered specific thyroid blood tests include TSH, Free T4, Total T3, and thyroid antibody levels. Since these tests are only snapshots of your thyroid function at one time, it is usually helpful to review the trends. Your physical exam and medical history become very important when there is a possibility of hypothalamic or pituitary disease because the blood tests are less reliable in those situations. Depending on your specific medical history the normal TSH ranges commonly reported may not truely represent normal thyroid function.
The laboratory evaluation of patients with suspected primary hypothyroidism relies heavily assumption on a normal functioning hypothalamus and pituitary. The differentiation of intrinsic thyroid disease from hypothyroidism due to diminished TSH secretion from hypothalamic or pituitary disease is a critical decision point. A low free thyroxine level along with a low or normal TSH level should generally lead to an evaluation for the possibility of failure of other endocrine systems that rely on normal pituitary function. One exception to this would be posthyperthyroid hypothyroidism where TSH values remain suppressed for months after the therapy. Primary hypothyroidism, the cause of hypothyroidism in 99% of all patients, is associated with a significant increase in the basal blood TSH concentration. One method for evaluating a patient with suspected primary hypothyroidism involves an initial TSH determination. The free T4 index is a marker of blood thyroid hormone levels based on testing of T4 and T3 concentrations in the blood. The presence of thyroid peroxidase antibodies (TPO antibodies) or thyroglobulin antibodies (TG antibodies) helps to distinguish among the causes of hypothyroidism and strongly suggests the presence of Hashimoto's thyroid disease.
This table, which is not exhaustive, reviews the possible diagnoses in patients with clinical low thyroid function, and it includes all causes of hypothyroidism. Note that the TSH can range from high to low depending on the specific cause of low thyroid function.
In a study from Norway (1), serum TSH within the reference range was positively and strongly associated with the risk of future hypothyroidism. The risk increased gradually from TSH of 0.50–1.4 mU/liter to TSH of 4.0–4.5 mU/liter. The likelihood of developing true hypothyroidism was defined as prescription of levothyroxine, or TSH above 4.50 mU/liter combined with free T4 below normal, in people without a history of hyperthyroidism. Conversely, TSH at the lower limit of the reference range may be associated with an increased risk of hyperthyroidism
1. Serum TSH within the Reference Range as a Predictor of Future Hypothyroidism and Hyperthyroidism: 11-Year Follow-Up of the HUNT Study in Norway The Journal of Clinical Endocrinology & Metabolism. January 1, 2012 vol. 97 no. 1 93-99