Thyroid Profile Interpretation: Hypothyroidism and Hyperthyroidism

The Hypothalamic-Pituitary-Thyroid (HPT) Axis - Why It Matters

The Tests in the Thyroid Profile

Total vs. Free: Total T4/T3 can be misleading because levels change when binding proteins change (e.g., elevated estrogen in pregnancy or on OCPs raises TBG, raising total T4 without true hyperthyroidism). Free T4 and free T3 are preferred because they reflect the biologically active fraction. - American Thyroid Association

Interpreting the Pattern: The Step-by-Step Approach

Step 1 - Check TSH First

• High TSH → thyroid is underperforming (hypothyroidism suspected)
• Low TSH → thyroid is overperforming (hyperthyroidism suspected)
• Normal TSH → euthyroid in the vast majority of cases

Step 2 - Check fT4 (and fT3 when needed) to classify severity and localize the defect

Pattern Recognition Table

Hypothyroidism - Detailed Interpretation

Overt Primary Hypothyroidism

• TSH: high + fT4: low
• The most common pattern. The thyroid gland is failing; low T4 removes feedback inhibition from the pituitary, which responds by producing more TSH.
• Common causes: Hashimoto's thyroiditis (autoimmune - most common cause worldwide in iodine-sufficient regions), post-thyroidectomy, post-radioiodine therapy, iodine deficiency, drugs (lithium, amiodarone).

Subclinical Hypothyroidism

• TSH: mildly elevated (4.0-10 mIU/L) + fT4: normal
• The gland is compensating; T4 is maintained at the cost of elevated TSH. Patients may be asymptomatic or have non-specific symptoms.
• Treatment is considered when TSH > 10, or if symptoms present, or in pregnancy.

Central (Secondary/Tertiary) Hypothyroidism

• TSH: low or normal + fT4: low
• Paradoxical - looks like hyperthyroidism (low TSH) but fT4 is also low.
• Cause: pituitary failure (hypopituitarism) or hypothalamic disease reducing TRH.
• This is why you must always check fT4 alongside TSH - a low TSH alone does not confirm hyperthyroidism without checking fT4.

T3 in Hypothyroidism

Hyperthyroidism - Detailed Interpretation

Overt Primary Hyperthyroidism

• TSH: low (usually < 0.1 mIU/L) + fT4: elevated ± fT3: elevated
• Excess thyroid hormone suppresses the pituitary, driving TSH to near-undetectable levels.
• Causes: Graves disease (~85% of cases), toxic multinodular goiter, toxic (hyperfunctioning) adenoma. - Robbins & Kumar Basic Pathology

T3 Toxicosis

• TSH: low + fT4: normal + fT3: elevated
• Some patients (particularly with early Graves or autonomous nodules) overproduce T3 disproportionately. A low TSH with normal fT4 should prompt free T3 measurement to avoid missing this diagnosis.

Subclinical Hyperthyroidism

• TSH: low + fT4: normal + fT3: normal
• The thyroid is marginally overactive, enough to suppress the pituitary but not raise peripheral hormones above the reference range.
• Important to recognize because it carries risk of atrial fibrillation and bone loss.

Secondary (Central) Hyperthyroidism

• TSH: normal or elevated + fT4: elevated + fT3: elevated
• Rare - caused by a TSH-secreting pituitary adenoma. The inappropriate "normal" or high TSH in the face of high thyroid hormones is the diagnostic clue.

Antibody Tests - Localizing the Cause

Common Pitfalls and Confounders

1. Biotin supplements artificially falsify results (usually falsely low TSH and falsely high fT4). Patients should stop biotin for at least 2 days before testing.
2. Pregnancy / oral contraceptives elevate TBG, raising total T4/T3 without true hyperthyroidism. Always use free hormone levels in these contexts.
3. Non-thyroidal illness (sick euthyroid syndrome): Severe illness lowers T3 (decreased peripheral conversion), may lower TSH and fT4. Interpret thyroid tests cautiously in acutely ill patients.
4. Amiodarone: Rich in iodine and structurally similar to T4. It can cause both hypo- and hyperthyroidism, and raises total T4 even in euthyroid patients.
5. Reverse T3 (rT3): Produced in states of illness/stress; biologically inactive. Measurement is not routinely useful.
6. Interference with "free" assays: Some assay platforms can be affected by heterophile antibodies or abnormal binding proteins, giving spuriously abnormal results.

Summary Diagnostic Algorithm

START: Measure TSH
        |
   ─────────────────────────────────
   |                               |
 HIGH TSH                        LOW TSH
   |                               |
Measure fT4                   Measure fT4 + fT3
   |                               |
fT4 low → OVERT PRIMARY HYPOTHYROIDISM     fT4 high or fT3 high → OVERT HYPERTHYROIDISM
fT4 normal → SUBCLINICAL HYPOTHYROIDISM    fT4 + fT3 normal → SUBCLINICAL HYPERTHYROIDISM
                                           fT4 low → CENTRAL HYPOTHYROIDISM
        |
  If TSH low/normal AND fT4 low → CENTRAL (secondary) HYPOTHYROIDISM

• Add anti-TPO to confirm autoimmune cause
• Add TRAb/TSI if hyperthyroidism (to confirm or exclude Graves)
• Consider radioactive iodine uptake scan or thyroid ultrasound for structural evaluation