SAT496 Tobacco Smoking May Mimic Subclinical Hyperthyroidism

Disclosure: R. Alshantti: None. U. Rafat: None. B. Alkhaurri: None. C.P. Barsano: None. M. Siddiqui: None. Introduction: Studies have shown lower levels of TSH in smokers and a positive association of smoking with hyperthyroidism. The urinary nicotine level is reported to be negatively correlated with TSH. Moreover, smoking cessation have shown reversible effects on thyroid functions and risk of having overt hypothyroidism increased more than 6-fold in the first 2 years after cessation of smoking. Whether transient smoking cessation, prior to blood work would alter thyroid functions is unknown. We report the case of an elderly woman who had normalization of suppressed TSH after smoking abstinence prior to her blood work, suggestive of smoking-induced alteration of the TSH level. Clinical Case: A 77- year-old woman was referred to endocrinology clinic for an abnormal TSH level. She had a suppressed TSH <0.01 uIU/mL [Ref range: 0.45-5.33], with normal free T4 0.78 ng/dl [Ref range: 0.70-2.70], and free T3 3.66 pg/ml [Ref range: 2.5-3.9], consistent with subclinical hyperthyroidism. Thyroid auto-antibodies were negative. She had a long-standing history of multinodular goiter with mild tracheal deviation for which she had declined surgical management. She was an active smoker with 40 pack years smoking history. There was no preceding viral illness. She was not taking any biotin-containing supplements or medications that would interfere with thyroid hormones. The patient denied symptoms of overt hyperthyroidism. On physical examination she had a large multinodular goiter with negative Pemberton’s sign. There was no exophthalmos, lid lag, tachycardia, tremors or hyperreflexia. Her subsequent blood tests, 2 and 4 months apart, were also consistent with subclinical hyperthyroidism with TSH levels of 0.115 to 0.144 uIU/mL with normal free T4 0.72 ng/dl and total T3 136.3 to 153 ng/dl [Ref range 80-187]. Based on her age, she met the treatment threshold. However, as the patient continued to smoke 10 cigarettes daily, even on the days when thyroid labs were drawn, treatment was deferred considering the possibility of smoking-induced altered thyroid function. Blood work re-drawn after a smoking abstinence of 10-12 hours showed normalization of TSH 0.58 uIU/mL consistent with a euthyroid state. Conclusion: When evaluating patients with abnormal thyroid functions, it is important to address smoking status. Cigarette smoking may alter thyroid function tests in a pattern consistent with subclinical or overt hyperthyroidism and its effect on thyroid functions may be transient and reversible. Therefore, the possibility of smoking-induced alteration of thyroid functions should be considered in active, clinically euthyroid smokers. It may be judicious to hold treatment with thionamides in such patients and to re-evaluate thyroid functions after smoking abstinence. Presentation Date: Saturday, June 17, 2023