Thyroid Endocrinology

Thyroid Endocrinology Chapter 9 in Randall et al. “Eckert Animal Physiology” Chapters 4 & 13 in Hadley “Endocrinology” Chapters 2, 7 & 8 in Norris “Vertebrate Endocrinology”

Major concepts covered in thyroid section: — Negative feedback regulation of hypothalamus-pituitary hormone secretion — Positive feedback actions of hormone — Multi-hormonal interactions in controlling a biological response — Local “non-endocrine” as well as endocrine regulation of hormone release — Parallel mechanisms of actions of thyroid hormones and steroid hormones (intracellular receptors) — General mechanisms of nuclear receptors and receptor action — Local activation of circulating hormones, another concept of “prohormone” — Plasma binding proteins for hormones

Some major functions of thyroid hormones in mammals — decrease cholesterol level in plasma — affect development of the nervous system, increase myelination — increase brain activity — increase glucose absorption by gut — increase basal metabolic rate and O2 consumption — increase heart rate — potentiates the action of growth hormone (permissive action of thyroid hormone) — other growth and differentiation effects

Some major functions of thyroid hormones in lower vertebrates — promotes growth and protein synthesis — influences food conversion in somatic growth — stimulates molting in snake and growth of feathers in birds (epidermal action) — stimulates parr-smolt transformation in salmonids and metamorphosis in amphibians

OH OH OH I I I I I O O O I I I I I CH 2 CH 2 CH 2 CH CH CH COOH COOH COOH NH NH NH 2 2 2 Thyroxine , T4 (tetraiodothyronine) (Major form released) 3,5,3’-triiodothyronine, T3 (More active than T4) 3,3’,5’-triiodothyronine, rT3 (reverse T3, inactive form) Several major kinds of thyroid hormones (all are iodinated thyronines or conjugated tyrosines)

Thyroid tissue - thyroid follicle Blood Capillary From Wikimedia commons

Basic unit for thyroid tissue - thyroid follicle Thyroid follicular cells (produces thyroid hormone and its precursor protein) Blood capillary Thyroid colloid (containing precursor protein that can form thyroid hormones)

Basic unit for thyroid tissue - thyroid follicle “Active” - thick columnar epithelium, colloid vacuolated “Inactive” - thin cuboidal epithelium, colloid “full”

Origin of Thyroid Gland — Thyroid hormone is the only iodine-containing hormone — Iodine concentrating cells are found even in invertebrates but most of these are cells exposed to the external environment — Vertebrate thyroid develops as an evagination of the floor of the oral epithelium behind the oral plate (thyroid diverticulum) — Subpharyngeal gland in larvel lamprey concentrates iodine and produces iodine-containing proteins and is open to the oral cavity via the subpharyngeal duct — Thyroid hormones might have originated as compounds released into the gut and then subsequently used as a hormone

1) thyroglobulin has to be synthesized OH OH tyrosines on thyroglobulin CH CH 2 2 CH CH Thyroid Hormone Synthesis and Release (1)

1) thyroglobulin has to be synthesized OH OH tyrosines on thyroglobulin CH CH 2 2 3) Conjugation of iodinated tyrosine residues to CH CH form iodinated thyronine (coupling reaction) OH Example shows two DI-Tyr I I conjugating to form tetraiodothyronine. 2) Iodination of Thyroid peroxidase tyrosine residues (organification of OH OH O iodine e.g., forming DI-Tyr) I I I I I I Thyroid peroxidase CH CH CH 2 2 2 CH CH CH Iodinated thyroglobulin Thyroid Hormone Synthesis and Release (2) Synthesis of iodinated thyronine via thyroid peroxidase action 4) Storage of iodinated thyroglobulin in the colloid

OH I I + O I I Fragments of CH 2 thyroglobulin CH COOH NH 2 Thyroxine , T4 (tetraiodothyronine) Thyroid Hormone Synthesis and Release (3) Stored iodinated thyroglobulin in colloid Reabsorption of colloid by phagocytotic Action of thyroid epithelial cells Enzymatic processing of reabsorbed thyroglobulin in thyroid epithelial cells Fragments recycled T4 can be released

Thyroid Hormone Synthesis and Release (4) — Several potential forms of iodinated thyronine can be synthesized in the thyroid — This depends on the conjugation of iodinated tyrosines on the thyroglobulin molecule DI-Tyr + DI-Tyr T4 MI-Tyr + DI-Tyr T3 or reverse T3 (3,3’,5’-triiodothyronine) MI-Tyr + MI-Tyr T2 forms MI-Tyr + Tyr T1 forms DI-Tyr + Tyr T2 forms

(–) TRH DA (–) (+) (–) TSH (+) T4 and T3 Neuroendocrine Control of Thyroid Gland Activity External & Internal Influences Brain Hypothalamus Long-loop -ve feedback Pituitary thyrotropes Thyroid Action on Target cell

TSH Na+/I- cotransport TSH receptor Na+/K+ ATPase Tg Pinocytosis & reuptake of colloid I I I I Tg Tg TSH ACTION ON THYROID BLOOD or TISSUE FLUID Na+ Na+ I- Release of T4 & T3 by diffusion ATP cAMP/PKA Pentose PO4 Pathway Peroxide Thyroglobulin (Tg) synthesis Free T4 (& T3) Thyroid Follicular Epithelial Cells vesicular transport of Tg to colloid Amino acids recycling MIT DIT & Tg fragments Degradation of Tg in lysosome Recycling of Free I Membrane Thyroid Peroxidase Activity COLLOIDSPACE Iodination of tyrosine & formation of iodothyronines Iodothyronines In Tg in colloid

Summary of Thyroid Hormone Synthesis From Wikimedia Commons

Changes in Thyroid Hormone Regulation During Metamorphosis in Frogs Stages of amphibian development

Neuroendocrine Control of Thyroid Gland Activity Just Prior To and At Metamorphosis in Amphibians (+) TRH DA (–) (+) (+) TSH (+) T4 and T3 External & Internal Influences Brain Hypothalamus Long-loop +ve feedback Pituitary thyrotropes Thyroid Action on Target cell E.g., gill & tail reabsorption

OH 3,5,3'-triiodotyronine, T3 I Outer Ring I (more active hormone) Deiodination Deiodinase I & II (Type I - liver, thyroid & kidney; II- brain, pituitary, placenta, BAT) O Other metabolites I I Inner Ring Deiodination 3,3',5'-triiodotyronine, rT3, reverse T3 CH 2 (inactive hormone form) CH COOH NH 2 Further Processing of Thyroxine Once Released from Thyroid Outer Ring Inner Ring Deiodinase III Thyroxine T4

— Thyroid hormone binding proteins also present in the blood. — Functional relevance and consequences?

Schematic of Nuclear Receptor Domains and Functions Hinge Hsp90-binding Leucine-rich Heptads Zinc-Fingers Hormone-Binding DNA-Binding

Nuclear Receptors Superfamily — includes receptors for steroids, thyroid hormones, vitamins, xenobiotics, and other “orphaned receptors” — these are intracellular phosphoproteins — these are DNA-binding transcription factors — DNA-binding is achieved through helices located at the base of the zinc-fingers

T box C CI D DR A (H) box CII DR Zn Zn CI N N T box A (H) box C Helix 2 Helix 2 Helix 1 Helix 1 Zn CII Zn D Linear (A) and 3-dimensional schematic (B) of DNA-binding domain region of nuclear receptor (Modified from Belanger “Molecular Endocrinology”) The two helices adjacent to the zinc-cysteines interaction folds upon one another to form a helix-turn-helix structure typical of transcription factors.

Idealized Hormone-response elements (HREs) as two half-sites (Consensus sequences; directionality indicated by arrows) n n n n n n A G A A C A T C T T G T T G T T C T A C A A G A (Glucocorticoid) n n n n n n A G G T C A T C C A G T T G A C C T A C T G G A (Estrogen) A G G T C A T C C A G T T G A C C T A C T G G A (Thyroid Hormone) n n n n n n A G G T C A T C C A G T A G G T C A T C C A G T (Vitamin D3)

Schematic of steroid hormone receptor action on transcription + + horm horm horm horm C C horm horm C C C C target gene HRE HRE Stabilization of receptor dimer and DNA binding Recruitment of adaptor proteins and interactions with nuclear transcription factors and transcription activators/regulators elements Regulation of gene transcription change in conformation and DNA-independent phosphorylation Receptor dimerization via Leu-rich region DNA binding & DNA-dependent hyperphosphorylation

BP Plasma binding protein Retinoids Thyroid hormone receptor Retinoid receptor legend RA RA DII Transcription Regulation Thyroid Hormone in Circulation & Mechanisms of Action BP Target Cell BP T3 BP Nucleus T4 T4 T3 T3 T3 T3 T3 T3 DI T4 T4 RNA protein mRNA Biological Effects

Note 1: — isoforms of nuclear receptors also present, e.g., TRa and TRb — tissue selective expression of receptor isoforms observed (e.g., heart expresses more TRa while liver expresses more TRb) — selectivity of actions may also result from this Note 2: — TR and RXR are also located in the mitochondria — some effects on mitochondrial DNA can also be exerted via these receptors — not all effects of TH on mitochondria are necessarily mediated by “mitochondria genomic action”

NOTE — although nuclear receptors mediate the action of membrane permeant steroid and thyroid hormones, these hormones are also known to bind to membrane receptors

Membrane Receptors for thyroid hormones — the presence of a modified nTR on plasma membrane has not been shown but thyroid hormones are also known to activate PLC via membrane actions — around 2002-2003, evidence suggest that TH can bind to integrin aVb3 which can activate PLC — this “receptor” is characterized by the ability to bind TETRAC (tetraiodothyroacetic acid,deamination product of T4) which acts here as an antagonist to block the actions of thyroid hormones on membrane ion fluxes, angiogenesis and cell proliferation

Non-endocrine Regulators of Thyroid Activity — iodine availability in food — insufficient iodine - TSH-induced hypertrophy and goiter — great excess of iodine - poisoning of the peroxidase enzyme — inhibitors of iodine uptake — thiocyanate - cabbage, Brussels sprouts, turnips, broccoli etc. contain thioglucosides which form thiocyanates upon digestion — other small negatively charged molecules can also compete for the uptake pump — other thio-compounds (e.g., thioamides) also inhibit the peroxidase enzyme & deiodinase

Thyroid Endocrinology