Thyroid-Stimulating Hormone (TSH)
TSH Reference Values in Pregnancy
TSH levels decrease in early pregnancy due to hCG-mediated thyroid stimulation, then gradually return toward nonpregnant values by late pregnancy.
| Units | Nonpregnant Adult | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| mIU/L | 0.4 – 4.0 | 0.1 – 2.5 | 0.2 – 3.0 | 0.3 – 3.0 |
Physiologic changes in pregnancy
- hCG stimulates the TSH receptor → transient suppression of TSH in 1st trimester.
- TSH slowly rises back toward baseline by 3rd trimester.
- Free T4 may be slightly higher early and normalize later.
- Total T4 increases due to higher TBG.
Causes of elevated TSH (hypothyroidism)
- Primary hypothyroidism (Hashimoto's disease)
- TSH-secreting pituitary tumor
- Thyroid hormone resistance
- Inadequate thyroid hormone replacement
- Adrenal insufficiency
- Iodine deficiency or excess
- Lithium or antithyroid drugs
Classification:
• Subclinical hypothyroidism: TSH 2.5–10 with normal FT4
• Overt hypothyroidism: TSH > trimester range + low FT4 OR TSH ≥10
Causes of low TSH (thyrotoxicosis)
- Graves disease
- hCG stimulation: early pregnancy, hyperemesis gravidarum, molar pregnancy/choriocarcinoma
- Exogenous thyroid hormone
- Thyroiditis
- Autonomous thyroid nodule
- Dopamine or glucocorticoid therapy
- Secondary hypothyroidism (pituitary dysfunction)
References
- Abbassi-Ghanavati M, Greer LG, Cunningham FG. Obstet Gynecol. 2009;114:1326–1331.
- Brent GA. Clin Obstet Gynecol. 1997;40(1):3–15. PMID: 9103946.
- Stagnaro-Green A et al. ATA Guidelines. Thyroid. 2011;21(10):1081–1125. PMID: 21787128.