Thyroid profile

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thyroid profile tests TSH T3 T4 normal values interpretation 2024

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Excellent - I now have comprehensive data from multiple authoritative textbooks. Here is the full thyroid profile overview:

Thyroid Profile

The thyroid profile (thyroid function tests) is a panel of blood tests used to evaluate the function of the thyroid gland. It typically includes TSH, free T4, free T3, and sometimes total T4/T3 and antibodies.

Hypothalamic-Pituitary-Thyroid (HPT) Axis

The HPT axis is the regulatory backbone of thyroid function:

Hypothalamus secretes TRH (thyrotropin-releasing hormone)
TRH stimulates the anterior pituitary to release TSH (thyroid-stimulating hormone)
TSH stimulates the thyroid gland to synthesize and release T3 and T4
T3/T4 exert negative feedback on both the pituitary (suppressing TSH) and the hypothalamus (suppressing TRH)
External factors: Cold and acute psychosis stimulate the axis; severe stress and corticoids/dopamine inhibit it

(Katzung's Basic and Clinical Pharmacology, 16th Ed)

The Tests - Normal Reference Ranges

Test	Normal Value	Hypothyroidism	Hyperthyroidism
Total T4	4.8-10.4 mcg/dL (62-134 nmol/L)	Low	High
Total T3	59-156 ng/dL (0.9-2.4 nmol/L)	Normal or Low	High
Free T4 (FT4)	0.8-1.4 ng/dL (10-18 pmol/L)	Low	High
Free T3 (FT3)	169-371 ng/dL (2.6-5.7 pmol/L)	Low	High
TSH	0.45-4.12 µIU/mL	High	Low
123I uptake (24h)	5-35%	Low	High
Anti-Tg antibodies	<200 IU/mL	Often present	Usually present
Anti-TPO antibodies (ATPO)	≤100 WHO units	Often present	Usually present
Serum thyroglobulin	Women: 1.5-38.5 mcg/L; Men: 1.4-29.2 mcg/L	Not indicated	Not indicated
TSI / TSH-R Ab	Negative (<140% of baseline)	Not indicated	Elevated in Graves' disease

(Katzung's Basic and Clinical Pharmacology, 16th Ed, p. 1077)

Individual Tests Explained

1. TSH (Thyroid-Stimulating Hormone)

The single most sensitive screening test for thyroid dysfunction
If TSH is normal, thyroid function is almost certainly normal
TSH is the first test to change in thyroid disease:
- High TSH = Primary hypothyroidism (thyroid under-producing, pituitary compensating)
- Low TSH = Hyperthyroidism or secondary/tertiary hypothyroidism
Reference: 0.45-4.12 µIU/mL (adults)
Pregnancy-specific ranges: 1st trimester 0.26-2.66; 2nd trimester 0.55-2.73; 3rd trimester 0.43-2.91 mIU/L

2. Free T4 (FT4) - Preferred over Total T4

Biologically active, unbound fraction of thyroxine
Not affected by changes in thyroid-binding globulin (TBG) unlike total T4
Total T4 is seldom measured nowadays - FT4 has largely replaced it
Reference: 0.8-1.4 ng/dL

3. Free T3 (FT3)

Active form; T4 is converted to T3 peripherally by deiodinases
More useful than total T3 in clinical practice
Helpful in T3 toxicosis (elevated T3 with normal T4)
Reference: 2.0-4.4 pg/mL

4. Total T4 and Total T3

Include both protein-bound and free fractions
Can be falsely elevated or depressed by changes in TBG (e.g., estrogen raises TBG, androgens lower TBG) without actual thyroid disease
Seldom used clinically now that free hormone assays are available

(Harper's Illustrated Biochemistry, 32nd Ed)

Interpreting the Pattern

TSH	FT4	FT3	Interpretation
High	Low	Low	Primary hypothyroidism
Low	High	High	Primary hyperthyroidism
Low/normal	Low	Low	Secondary (pituitary) or tertiary (hypothalamic) hypothyroidism
Low	Normal	Normal	Subclinical hyperthyroidism
High	Normal	Normal	Subclinical hypothyroidism
Normal	Low (total only)	-	Low TBG state (androgens, nephrotic syndrome)
Normal	High (total only)	-	High TBG state (estrogens, pregnancy, OCP)

Thyroid Antibodies

Antibody	Significance
Anti-TPO (thyroid peroxidase)	Hashimoto's thyroiditis, Graves' disease; ≤100 WHO units normal
Anti-thyroglobulin (Anti-Tg)	Hashimoto's, thyroid cancer monitoring; <200 IU/mL normal
TSH receptor antibody (TSI / TRAb)	Specific for Graves' disease; causes hyperthyroidism by mimicking TSH

Drug Effects on Thyroid Tests

Drugs can cause both in vitro and in vivo effects - this is a common source of misinterpretation (Henry's Clinical Diagnosis and Management by Laboratory Methods; Washington Manual of Medical Therapeutics):

Drugs causing decreased T4/T3 (true hypothyroidism - TSH elevated):

Iodine (amiodarone, radiographic contrast), lithium, some tyrosine kinase inhibitors, immune checkpoint inhibitors

Drugs suppressing TSH without true hypothyroidism:

Glucocorticoids (high dose), dopamine - suppress TSH secretion

Drugs affecting TBG (TSH remains normal):

Increase TBG (raise total T4/T3 but not free): Estrogens, tamoxifen, raloxifene, methadone, heroin, SERMs
Decrease TBG (lower total T4/T3 but not free): Androgens

Drugs displacing T4 from binding proteins (TSH normal):

Furosemide (high dose), salicylates, phenytoin, carbamazepine

Drugs inhibiting T4 → T3 conversion:

Amiodarone (complex effects - can cause hypo- or hyperthyroidism), glucocorticoids, high-dose propranolol (useful in thyroid storm)

Drugs causing lab artifact only:

Biotin - interferes with immunoassays, causing false results without real thyroid dysfunction

Key Clinical Points

Start with TSH - it is the most sensitive marker and often sufficient for screening
Use FT4 (not total T4) for confirming thyroid hormone levels
Add FT3 when T3 toxicosis is suspected or when monitoring hyperthyroidism treatment
TSI/TRAb confirms Graves' disease
Anti-TPO is the best antibody for diagnosing Hashimoto's thyroiditis
Thyroglobulin is used as a tumor marker after thyroidectomy for differentiated thyroid cancer
Always check the medication list before interpreting any thyroid test - many common drugs interfere

Sources: Katzung's Basic and Clinical Pharmacology, 16th Ed | Harper's Illustrated Biochemistry, 32nd Ed | Henry's Clinical Diagnosis and Management by Laboratory Methods | Washington Manual of Medical Therapeutics | Goodman & Gilman's Pharmacological Basis of Therapeutics

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