radioactive iodine uptake and thyrotoxicosis factitia

Q; 32 years old female with previous history of Anorexia Nervosa presents with signs and symptoms of thyrotoxicosis. 'Thyrotoxicosis factitia' is highly suspected as no exophthalmos was noted. A low TSH level is found, along with normal free T4 and T3 levels. 24-hour radioiodine uptake ordered. Which of the following may help in confirming the diagnosis?

A) High 24-hour radioiodine uptake

B) Low 24-hour radioiodine uptake

Answer: B

There are many clinical situations, besides the intentional or unintentional intake of exogenous thyroid hormone, that may cause thyrotoxicosis factitia. It includes thyroid cancer, which may require suppressive doses of thyroxine to minimize TSH stimulation of tumor growth, and a higher-than-usual dose prescribed to shrink the thyroid gland. Also, psychiatric disorders are prone to low levels of TSH, and thyroid hormone may be prescribed in combination with antipsychotics. 

Besides history and thyroid hormone levels, the 24-hour radioiodine uptake in the thyroid is nearly absent in patients with exogenous hyperthyroidism due to the suppression of TSH secretion. The only caveat to this confirmatory test is the presence of autonomous functioning thyroid tissue elsewhere in the body, or thyroiditis with the release of preformed hormone from any inflamed gland, such as struma ovarii, and due to iodine-induced hyperthyroidism. 

#endocrinology

#nuclear-medicine

References:

1. Cohen JH 3rd, Ingbar SH, Braverman LE. Thyrotoxicosis due to ingestion of excess thyroid hormone. Endocr Rev 1989; 10:113.

2. Vorasart P, Sriphrapradang C. Factitious thyrotoxicosis: how to find it. Diagnosis (Berl). 2020 May 26;7(2):141-145. doi: 10.1515/dx-2019-0015. PMID: 30927742.

3. Ross DS. Syndromes of thyrotoxicosis with low radioactive iodine uptake. Endocrinol Metab Clin North Am. 1998 Mar;27(1):169-85. doi: 10.1016/s0889-8529(05)70305-4. PMID: 9534035.