Thyroid Stimulating Hormone (TSH)
- Thyroid stimulating hormone (TSH) is secreted by the anterior pituitary, in response to the stimulating effect of thyroid releasing hormone (TRH) from the hypothalamus. TSH acts on the thyroid gland to stimulate release of T3 and T4.
- The release of TSH is suppressed by circulating thyroid hormone, and its levels are affected by small changes in circulating T3 and T4 such that TSH changes in a logarithmic scale compared to the linear scale of the thyroid hormones.
- TSH is a sensitive marker of thyroid function, and the first line test for assessment of thyroid dysfunction.
- 0.4 - 4.1 mIU/L
- Reduced TSH - suggestive of primary hyperthyroidism
- Elevated TSH - suggestive of primary hypothyroidism
- After treatment for thyrotoxicosis, TSH may remain suppressed even once the T4 has returned to normal.
- Unwell patients with nonthyroidal illness may have a reduced TSH, while the TSH may elevate during the recovery phase of the illness.
- In patients with pituitary or hypothalamic pathology (central hyper or hypothyroidism) the TSH will be deranged in the same direction as the thyroid hormones.
- Resistance to thyroid hormone or TSH may result in an elevated TSH.
- The presence of heterophile antibodies may result in falsely elevated or reduced TSH levels.
- A suppressed TSH is suggestive of primary hyperthyroidism.
Causes of Reduced TSH
- Grave's disease
- Toxic multinodule goitre
- Toxic nodule
- Thyroiditis - postviral, postpartum, lymphocytic
- Drugs - thyroxine, amiodarone, iodine
- Pregnancy-related - hyperemesis gravidarum, hydatidiform mole
- Struma ovarii
- Congenital hyperthyroidism
- Subclinical hyperthyroidism
- Nonthyroidal illness
- Central hypothyroidism
- Congenital TSH or TRH deficiency
- Supressed TSH with elevated T4 - suggestive of primary hyperthyroidism
- Suppressed TSH with normal T4 - suggestive of subclinical hyperthyroidism
- Suppressed TSH with reduced T4 - suggestive of nonthyroidal illness or central hyperthyroidism
- An elevated TSH is suggestive of primary hypothyroidism.
Causes of Elevated TSH
- Autoimmune thyroiditis - Hashimoto’s, atrophic
- Hypothyroid phase of thyroiditis
- Iatrogenic - radioiodine therapy, thyroidectomy, external irradiation of the neck
- Drugs - amiodarone, lithium, interferons, interleukin-2, iodide
- Iodine deficiency
- Thyroid infiltration - amyloidosis, tumour
- Congenital hypothyroidism
- Subclinical hypothyroidism
- Nonthyroidal illness (recovery phase)
- Central hyperthyroidism
- TSH receptor defects
- TSH resistance
- Elevated TSH with reduced T4 - consistent with primary hypothyroidism
- Elevated TSH with normal T4 - suggestive of subclinical hypothyroidism
- Elevated TSH with elevated T4 - suggestive of central hyperthyroidism