What is the best course of action for a patient with a suppressed Thyroid-Stimulating Hormone (TSH) level and symptoms of hyperthyroidism, including diarrhea?

Immediate Methimazole Initiation is Indicated for TSH <0.1 with Diarrhea

Yes, start methimazole immediately for a patient with TSH <0.1 and diarrhea, as this represents biochemically confirmed hyperthyroidism requiring urgent treatment to prevent progression to thyroid storm and reduce morbidity from thyrotoxicosis. 1, 2

Diagnostic Confirmation

Biochemical hyperthyroidism is confirmed when TSH is suppressed (<0.1 mIU/L) with elevated free T4 and/or free T3 levels. 2, 3 The diarrhea is a classic manifestation of thyrotoxicosis and may be partially due to bile acid malabsorption, which resolves with control of hyperthyroidism. 4

• Measure free T4 and free T3 immediately alongside the suppressed TSH to confirm overt hyperthyroidism versus subclinical disease 2, 3
• Check TSH-receptor antibodies and thyroid peroxidase antibodies to distinguish Graves' disease from toxic nodular goiter 2, 3
• Obtain thyroid ultrasound to assess for nodular disease or diffuse enlargement 2, 3
• Consider thyroid scintigraphy if autonomy (toxic nodular goiter) is suspected based on clinical presentation 3

Treatment Algorithm

Methimazole is the preferred antithyroid drug for treating hyperthyroidism in most patients, as it inhibits thyroid hormone synthesis and controls the hypermetabolic state. 5, 1, 6

Immediate Management

• Start methimazole 10-20 mg daily for mild hyperthyroidism, or 30-40 mg daily for severe hyperthyroidism 1, 6
• Add propranolol 20-40 mg three to four times daily to control tachycardia, tremor, and other adrenergic symptoms until thyroid hormone levels normalize 7, 1
• The beta blocker provides immediate symptomatic relief while methimazole takes 3-6 weeks to reduce thyroid hormone levels 7

Monitoring Protocol

• Monitor free T4 and free T3 every 2-4 weeks initially, as these reflect current thyroid status more accurately than TSH during treatment 8
• TSH remains suppressed for months even after thyroid hormones normalize, making it unreliable for early treatment monitoring 8
• Check complete blood count before starting methimazole and if fever or sore throat develop, as agranulocytosis is a serious side effect requiring immediate drug discontinuation 7

Critical Safety Considerations

Watch for agranulocytosis, which presents with sore throat and fever—obtain CBC immediately and discontinue methimazole if this occurs. 7 Other side effects include hepatitis, vasculitis, and thrombocytopenia. 7

Never delay treatment waiting for antibody results or imaging, as untreated hyperthyroidism carries significant cardiovascular and metabolic risks. 1, 2 The diarrhea, combined with TSH <0.1, indicates active thyrotoxicosis requiring immediate intervention to prevent complications including atrial fibrillation, thyroid storm, and progressive weight loss. 2, 4

Disease-Specific Considerations

If Graves' Disease (70% of hyperthyroidism cases)

• Continue methimazole for 12-18 months with goal of inducing remission 2, 6
• Approximately 50% of patients experience recurrence after stopping antithyroid drugs 2
• Risk factors for recurrence include age <40 years, free T4 ≥40 pmol/L, TSH-binding inhibitory immunoglobulins >6 U/L, and goiter size ≥WHO grade 2 2
• Consider long-term treatment (5-10 years) for patients at high risk of recurrence, which reduces recurrence rates to 15% 2

If Toxic Nodular Goiter (16% of cases)

• Methimazole provides temporary control but does not cure autonomous nodules 6
• Definitive treatment with radioiodine or thyroidectomy is typically required 6
• Antithyroid drugs are used to achieve euthyroid state before definitive therapy 6

Common Pitfalls to Avoid

• Do not wait for euthyroidism before addressing diarrhea—the diarrhea will improve as hyperthyroidism is controlled, with bile acid absorption normalizing alongside thyroid function 4
• Do not use TSH alone for treatment monitoring in the first 3-6 months—TSH remains suppressed long after free T4/T3 normalize 8
• Do not assume subclinical hyperthyroidism if diarrhea is present—symptomatic patients with TSH <0.1 almost always have elevated thyroid hormones requiring treatment 1, 2
• Do not overlook pregnancy status—methimazole is contraindicated in first trimester due to teratogenicity; propylthiouracil is preferred if pregnant 7