thyroid gland
Summary
The thyroid is a neck endocrine gland that secretes T3 and T4, which raise basal metabolic rate, and calcitonin, which lowers blood calcium. Most testable: the TRH → TSH → T3/T4 negative feedback loop, where low T4 with high TSH means a thyroid defect.
Detail
Meaning: The thyroid is an endocrine gland in the neck that makes thyroxine (T4) and triiodothyronine (T3), iodine-containing hormones derived from tyrosine. They raise basal metabolic rate by increasing O₂ use, ATP turnover, and heat production, and they support growth and nervous system development. Its parafollicular (C) cells secrete calcitonin, which lowers blood Ca²⁺ and opposes parathyroid hormone.
On the MCAT: Control is a negative feedback loop: hypothalamus releases TRH → anterior pituitary releases TSH → thyroid releases T3 and T4, which then inhibit TRH and TSH release. Passages give hormone levels and ask where the defect is. Low T4 with high TSH points to the thyroid itself, because feedback inhibition is lost. Low T4 with low TSH points to the pituitary or hypothalamus. Iodine deficiency lowers T4, raises TSH, and enlarges the gland (goiter). Excess thyroid hormone raises heart rate, body temperature, and metabolic rate; deficiency does the opposite.
Common confusion: T3 and T4 come from an amino acid, but they do not act like water-soluble peptide hormones. They are lipophilic, enter cells, bind intracellular receptors, and change gene transcription, like steroid hormones. Also, T3 and T4 do not regulate calcium. Calcitonin (from the thyroid) and PTH (from the parathyroids) do.
Sources
- Campbell Biology
- Principles of Anatomy and Physiology
- Human Physiology: An Integrated Approach
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.