What is the initial treatment for a patient newly diagnosed with hyperthyroidism?

Laboratory Workup for Newly Diagnosed Hyperthyroidism

The essential initial laboratory workup for newly diagnosed hyperthyroidism includes TSH and free T4 (FT4), with T3 added if symptoms are severe despite minimal FT4 elevation, and TSH receptor antibodies (TSH-R-Ab) if Graves' disease is suspected based on clinical features. 1

Core Laboratory Tests

Primary Screening Tests

• TSH and Free T4 (FT4) are the fundamental tests for confirming hyperthyroidism and establishing the diagnosis 1
• T3 measurement should be added in highly symptomatic patients with minimal FT4 elevations, as some patients may have T3 toxicosis 1
• These tests differentiate between overt hyperthyroidism (suppressed TSH with elevated FT4/T3) and subclinical hyperthyroidism (suppressed TSH with normal FT4/T3) 2

Etiologic Workup

• TSH receptor antibody (TSH-R-Ab) testing is recommended when clinical features suggest Graves' disease, particularly if ophthalmopathy or T3 toxicosis is present 1, 3
• TSH-R-Ab measurement aids in accurate diagnosis and differential diagnosis, and helps predict treatment response 3
• If TSH-R-Ab remains elevated above 10 mU/L after 6 months of antithyroid drug treatment, remission is unlikely and definitive therapy should be considered 4

Additional Baseline Tests

• Complete blood count (CBC) with differential should be obtained before initiating antithyroid drugs to establish baseline white blood cell count, as agranulocytosis is a serious potential complication 5, 6
• Liver function tests (ALT/AST, bilirubin, alkaline phosphatase) are essential baseline measurements given the hepatotoxicity risk, particularly with propylthiouracil 5
• Prothrombin time (PT/INR) monitoring should be considered, as hyperthyroidism may cause hypoprothrombinemia and bleeding, especially before surgical procedures 5, 6

Pattern Recognition for Differential Diagnosis

Distinguishing Central from Primary Hyperthyroidism

• Low TSH with low FT4 indicates central hypothyroidism (not hyperthyroidism) and requires evaluation for hypophysitis 1
• This pattern should prompt evaluation of morning ACTH and cortisol levels, basic metabolic panel, and consideration of pituitary imaging 1

Clinical Context Matters

• Physical examination findings of ophthalmopathy or thyroid bruit are diagnostic of Graves' disease and warrant early endocrine referral 1
• In the absence of these findings, thyroid scintigraphy may be needed if thyroid nodules are present or the etiology remains unclear 2

Monitoring Strategy

Initial Phase

• After diagnosis, thyroid function tests should be monitored every 2-3 weeks to detect transition from hyperthyroidism to hypothyroidism, which commonly occurs with transient thyroiditis 1
• For patients on antithyroid drugs, repeat TSH and free T4 testing after 6-8 weeks to adjust medication dosing 1

Treatment-Specific Monitoring

• TSH-R-Ab should be repeated at 12-18 months in patients on methimazole to guide decisions about continuing or stopping treatment 3
• Persistently elevated TSH-R-Ab indicates need for continued treatment or consideration of definitive therapy with radioactive iodine or surgery 3, 4

Critical Safety Monitoring

During Antithyroid Drug Therapy

• Patients must be counseled to immediately report sore throat, fever, skin eruptions, or general malaise, which may indicate agranulocytosis requiring urgent CBC with differential 5, 6
• Hepatic function monitoring is particularly important in the first 6 months, with patients instructed to report anorexia, pruritus, jaundice, light-colored stools, dark urine, or right upper quadrant pain 5
• Symptoms of vasculitis (new rash, hematuria, decreased urine output, dyspnea, hemoptysis) require immediate evaluation, as severe complications and death have occurred 5, 6

Pre-Treatment Considerations

• In pregnant women or those planning pregnancy, thyroid function tests and treatment approach must be adjusted, with propylthiouracil preferred in the first trimester and methimazole in the second and third trimesters 1, 5, 6, 3
• Baseline cardiovascular assessment is important, as hyperthyroidism increases cardiac workload and can precipitate heart failure in patients with underlying cardiac disease 1