January 2010 Answer
A) Primary hyperthyroidism
- A very low TSH is almost always caused by either primary hyperthyroidism (suppression via high free thyroid hormone) or secondary hypothyroidism (decreased pituitary function).
- In rare cases, it can be induced by medications. In this patient, high levels of T3and free T4are causing the low TSH, indicating primary hyperthyroidism.
- In secondary hyperthyroidism, the TSH is elevated in addition to at least the T3.
- Patients with an increased thyroxine-binding protein level have an increase in T3but not free T4or TSH and also have an increased unsaturated thyroxine-binding globulin (UTBG).
- Patients with euthyroidsick syndrome usually have a low T3or T4, but a normal or slightly elevated TSH.
Harr, R.R. (2007). Clinical Laboratory Science Review, 3rded. [F.A. Davis Company pg 271]