SAT-384 TSH Secreting Adenoma vs. Thyroid Hormone Resistance

Abstract Disclosure: A. Patel: None. S. Yavuz: None. Introduction: This case describes a patient with markedly elevated free T4 and total T3, with a normal TSH. It illustrates some of the workup that is needed to distinguish between two possible conditions. Case Description: This is a 76 year old man with a past medical history of atrial flutter (s/p ablation), sinus node dysfunction (s/p pacemaker), COPD (no recent steroid use), HTN, & HLD who was referred by his primary due to abnormalities in routine thyroid function tests. The patient endorsed occasional diarrhea and some shakiness of his hands. His weight was stable. He denied palpitations, neck pain/swelling, dysphagia, hoarseness, heat sensitivity, biotin intake, recent IV contrast, or recent infection. His labs at the referral: TSH 1.23, free T4 >7.7, total T3 >651, thyroglobulin (Tg) antibody level of 1123, and thyroid peroxidase antibody level >600. No prior thyroid tests had been done. Our own labs 18 days later showed a TSH of 1.70, free T4 >7.7, total T4 >24.9, total T3 >651, TRAb <1, TSI <89%, Tg antibody <1, and Tg of 5.9. Further tests were then obtained. Alpha subunit was 0.3, and the alpha subunit:TSH molar ratio was calculated to be 0.17647. SHBG was 14. Thyroid ultrasound showed no abnormal findings. A pituitary MRI couldn’t be done due to incompatibility with the patient’s pacemaker. A CT Head with IV contrast couldn’t be done due to discovering that the patient had an AKI. Finally, a TSH with HAMA washout was sent, but is still currently pending. The patient is doing well overall and has not clinically changed. His AKI is thought to be due to recent addition of diuretics for lower extremity edema as well as NSAID use for joint pains. Discussion: In his first visit, we repeated TFTs to ensure this was not the hyperthyroid phase of painless thyroiditis as both TPO antibodies and Tg were positive; normally the TSH would be suppressed but it does typically lag behind changes in T4 and T3. Graves was excluded with negative TRAb and TSI. A total T4 was also obtained to rule out any condition in which there is decreased T4 binding to TBG, which would result in high free T4 with low total T4; both salicylates and certain NSAIDs can do this (the patient was taking aspirin and meloxicam). The next few tests were ordered specifically to differentiate between a TSH-secreting adenoma and Resistance to Thyroid Hormone. Alpha subunit was 0.3 and the alpha subunit:TSH molar ratio was 0.17647, both of which are normal; they would usually both be elevated in patients with a TSH-secreting adenoma. The SHBG was low at 14; it’s typically high in patients with a TSH-secreting adenoma. The patient had an AKI so brain imaging was postponed for now. The TSH with HAMA washout is pending at the time of this submission. In conclusion, the more likely diagnosis therefore seems to be Resistance to Thyroid Hormone. Further diagnostic testing will be done to confirm that. Presentation: Saturday, July 12, 2025