About

Thyroid health is pivotal for metabolic regulation, yet interpreting lab results can be complex due to the inverse relationship between Thyroid Stimulating Hormone (TSH) and thyroid hormones (T3/T4). This complexity increases significantly during pregnancy, where the demand for thyroid hormones changes to support fetal development.

This tool aids in the preliminary screening of thyroid function by comparing patient values against demographic-specific reference ranges. Accuracy is critical here; for example, TSH ranges that are considered normal for an adult non-pregnant woman may suggest hypothyroidism in the first trimester of pregnancy, potentially impacting fetal neurodevelopment. This calculator differentiates between Overt and Subclinical states, providing a nuanced view often required in endocrinology and obstetrics.

Formulas

The assessment of thyroid function relies on the negative feedback loop between the pituitary gland and the thyroid. The core relationships are modeled as follows:

Status ↔ f(TSH, FT4, FT3)

Primary Hypothyroidism Logic:

TSH > Ref_max ∩ FT4 < Ref_min

Subclinical Hypothyroidism:

TSH > Ref_max ∩ FT4 ∈ [Ref_min, Ref_max]

Primary Hyperthyroidism:

TSH < Ref_min ∩ (FT4 > Ref_max ∪ FT3 > Ref_max)

Reference Data

Demographic / Condition	TSH Reference (mIU/L)	Free T4 Reference (ng/dL)	Clinical Significance
Adult (General)	0.40 – 4.00	0.8 – 1.8	Standard baseline for metabolic health.
Pregnancy: 1st Trimester	0.10 – 2.50	0.9 – 2.2	Lower TSH due to hCG cross-reactivity.
Pregnancy: 2nd Trimester	0.20 – 3.00	0.8 – 2.0	TSH rises slightly as hCG plateaus.
Pregnancy: 3rd Trimester	0.30 – 3.00	0.8 – 1.8	Ranges normalize towards non-pregnant limits.
Children (1-5 Years)	0.70 – 6.00	0.9 – 2.0	Higher TSH is physiological in growth phases.
Children (6-14 Years)	0.60 – 5.00	0.8 – 1.9	Gradual shift towards adult reference ranges.
Elderly (>70 Years)	0.40 – 6.00	0.7 – 1.7	Physiological shift; treatment targets differ.

Frequently Asked Questions

During early pregnancy, the hormone hCG (Human Chorionic Gonadotropin) stimulates the thyroid gland similarly to TSH. This causes free T4 levels to rise and TSH levels to drop. Consequently, the upper limit for 'normal' TSH is lower in the first trimester (usually ~2.5 mIU/L) compared to non-pregnant adults.

This condition occurs when your TSH levels are elevated above the reference range, but your T4 and T3 levels remain within normal limits. It is a mild form of thyroid failure that may or may not require treatment depending on symptoms, age, and pregnancy status.

This tool is designed for Free T4 (FT4) and Free T3 (FT3). Total T4 levels are heavily influenced by binding proteins like TBG (Thyroxine Binding Globulin), which increase during pregnancy or estrogen use, potentially skewing interpretation without correction.

Euthyroid refers to a normally functioning thyroid gland where TSH, T4, and T3 levels all fall within the established reference ranges for the specific demographic or physiological state.