How can I interpret thyroid‑stimulating hormone (TSH) results to determine if a patient has primary hypothyroidism or primary hyperthyroidism?

Interpreting TSH to Determine Hypothyroidism vs. Hyperthyroidism

TSH moves in the opposite direction of thyroid function: elevated TSH indicates hypothyroidism, while suppressed TSH indicates hyperthyroidism. This inverse relationship exists because TSH is produced by the pituitary to stimulate the thyroid gland—when thyroid hormone is low, TSH rises to compensate; when thyroid hormone is excessive, TSH is suppressed 1.

Understanding the TSH-Thyroid Hormone Relationship

TSH is the most sensitive initial screening test for thyroid dysfunction, with sensitivity of approximately 98% and specificity of 92% 2, 3. The normal reference range is typically 0.45-4.5 mIU/L, though this varies slightly by laboratory 1.

Primary Hypothyroidism (Thyroid Gland Failure)

Elevated TSH with low or normal free T4 indicates primary hypothyroidism 4, 5:

• TSH >10 mIU/L with normal free T4 = subclinical hypothyroidism requiring treatment 6, 4
• TSH >6.5 mIU/L is generally considered elevated 1
• TSH elevated with low free T4 = overt hypothyroidism requiring immediate treatment 6, 5

The pituitary detects insufficient thyroid hormone and increases TSH production to stimulate the failing thyroid gland 4, 5.

Primary Hyperthyroidism (Thyroid Overactivity)

Suppressed TSH with elevated or normal free T4/T3 indicates primary hyperthyroidism 2, 7:

• TSH <0.1 mIU/L is considered definitively low 1, 2
• TSH <0.1 mIU/L with elevated free T4 or T3 = overt hyperthyroidism 2, 8
• TSH <0.1 mIU/L with normal free T4 and T3 = subclinical hyperthyroidism 1, 2

The pituitary suppresses TSH production because excessive thyroid hormone is already circulating 2, 7.

Diagnostic Algorithm

Step 1: Measure TSH First

Always begin with TSH as your initial screening test 2, 3. TSH alone can identify most thyroid dysfunction cases 2, 3.

Step 2: Confirm with Free T4 (and Free T3 if Needed)

When TSH is abnormal, measure free T4 to distinguish subclinical from overt disease 6, 2, 3:

• If TSH is elevated: Measure free T4 to differentiate subclinical hypothyroidism (normal T4) from overt hypothyroidism (low T4) 6, 4
• If TSH is suppressed: Measure free T4 and free T3 to differentiate subclinical hyperthyroidism (normal hormones) from overt hyperthyroidism (elevated hormones) 2, 7, 8

Step 3: Confirm Persistence

Repeat testing after 3-6 weeks before making treatment decisions, as 30-60% of mildly abnormal TSH values normalize spontaneously 6, 2. TSH can vary by up to 50% day-to-day due to physiological factors 2.

Critical Pitfalls to Avoid

Don't Rely on Single TSH Values

Never make treatment decisions based on a single abnormal TSH measurement 6, 2. Transient elevations or suppressions occur with acute illness, medications (iodine, dopamine, glucocorticoids), recent iodine exposure, or recovery from thyroiditis 6, 2.

Don't Miss Central Hypothyroidism

In central (secondary) hypothyroidism, TSH may be low, normal, or even slightly elevated despite low free T4 4, 5, 3. This occurs when the pituitary or hypothalamus fails, so TSH cannot be used as a reliable screening test 6, 3. Always check free T4 alongside TSH in patients with pituitary disease or symptoms despite normal TSH 6, 3.

Recognize Age-Related Changes

The normal TSH reference range shifts upward with advancing age, reaching 7.5 mIU/L in patients over 80 6. Standard population reference ranges may be inappropriate for elderly patients 6.

Account for Non-Thyroidal Illness

Acute illness, hospitalization, and severe non-thyroidal disease can transiently suppress TSH or alter thyroid hormone levels 6, 2. Recheck TSH and free T4 4-6 weeks after recovery from acute illness before diagnosing thyroid dysfunction 6.

Summary Table

TSH Level	Free T4	Free T3	Diagnosis
>10 mIU/L	Normal	Normal	Subclinical hypothyroidism (treat) [6,4]
>6.5 mIU/L	Low	Low/Normal	Overt hypothyroidism (treat immediately) [1,6,5]
<0.1 mIU/L	Elevated	Elevated	Overt hyperthyroidism [2,7]
<0.1 mIU/L	Normal	Normal	Subclinical hyperthyroidism [1,2]
Low/Normal	Low	Low	Central hypothyroidism (pituitary/hypothalamic) [4,5,3]

The key principle: TSH and thyroid hormone levels move in opposite directions in primary thyroid disease, but this relationship breaks down in central hypothyroidism 4, 5, 3.