Overt Hyperthyroidism (Thyrotoxicosis)
Low TSH with elevated T4 and T3 indicates overt hyperthyroidism, and you must determine the underlying cause through radioactive iodine uptake (RAIU) testing unless the clinical presentation clearly indicates Graves' disease. 1
Diagnostic Approach
The combination of suppressed TSH (<0.01 mU/L) with elevated free T4 and T3 confirms overt thyrotoxicosis 1. This biochemical pattern requires systematic evaluation to identify the etiology and guide treatment.
When RAIU Testing is NOT Needed
Skip RAIU if the patient presents with:
- Symmetrically enlarged thyroid gland
- Recent onset ophthalmopathy (eye signs)
- Moderate to severe hyperthyroidism symptoms
This clinical triad makes Graves' disease sufficiently likely that further diagnostic workup is unnecessary 1.
When RAIU Testing IS Required
Perform RAIU (with thyroid scan if nodules present) when the diagnosis is uncertain 1. The uptake pattern distinguishes between:
Elevated/Normal RAIU:
- Graves' disease (diffuse uptake)
- Toxic adenoma or toxic multinodular goiter (focal uptake)
- TSH-producing pituitary adenoma (rare)
- Thyroid hormone resistance (rare)
Near-Absent RAIU:
- Painless (silent) thyroiditis
- Subacute thyroiditis
- Amiodarone-induced thyroiditis
- Exogenous thyroid hormone ingestion
Critical Exclusions Before Diagnosis
Before confirming hyperthyroidism, you must exclude assay interference and rare conditions when TSH is not appropriately suppressed 1, 2:
- Euthyroid hyperthyroxinemia: Elevated thyroid hormone binding proteins (TBG excess from pregnancy, estrogen, hepatitis), familial dysalbuminemia, or heterophile antibodies
- TSH-producing pituitary adenoma: Look for pituitary lesion on MRI and elevated alpha-subunit levels 1
- Thyroid hormone resistance: Requires family history and genetic testing for T3-receptor mutations 1
- Biotin interference: Recent high-dose biotin supplementation can falsely elevate thyroid hormone measurements 2, 3
Treatment Framework
Treatment depends entirely on the underlying cause identified by RAIU 1:
For Graves' Disease (Most Common)
- Antithyroid drugs (methimazole preferred except in first trimester pregnancy when propylthiouracil is used)
- Radioactive iodine ablation (contraindicated in pregnancy)
- Thyroidectomy (by high-volume thyroid surgeon)
For Toxic Nodular Disease
- Radioactive iodine or surgery are definitive options
- Antithyroid drugs provide temporary control but not cure
For Thyroiditis (Low RAIU)
- Supportive care only - self-limited condition
- Beta-blockers for symptom control
- No antithyroid drugs (thyroid not overproducing hormone, just releasing stored hormone)
Common Pitfalls
Do not treat transient TSH suppression without confirming persistent abnormality - repeat testing in 4-6 weeks if clinical suspicion is low 1.
Do not start antithyroid drugs empirically before determining etiology - treating thyroiditis with antithyroid medications is inappropriate and ineffective since the gland is not actively synthesizing excess hormone 1.
In pregnancy, never use radioactive iodine for diagnosis or treatment - rely on clinical presentation and TSH/thyroid hormone levels with trimester-specific reference ranges 1.
The severity of symptoms correlates only moderately with hormone elevation 1, so assess cardiovascular and neuromuscular complications independently when formulating your treatment plan, particularly in elderly patients and those with cardiac risk factors.